Combined cycle electric power plant with a steam turbine having an improved valve control system

ABSTRACT

A combined cycle electric power plant includes two gas turbines, a steam turbine, and a digital control system with an analog or manual backup. Each of the gas turbines has an exhaust heat recovery steam generator connected to a common header from which the steam is supplied by one or both of the steam generators for operating the steam turbine. Both the digital and the analog systems provide a digital input to an interface for controlling the steam turbine valves. The analog system is controlled to operate a respective valve by an input to its interface which determines valve position in accordance with its duration. The digital system is controlled to operate a respective valve by an input to the interface in accordance with the repetitive duration of the signal. The analog system input and digital system input is applied to an interface for each valve. A plurality of the valves are operated singly through parallel connected interfaces in response to plant conditions, and a plurality of the valves are operated sequentially through respective individual disconnected interfaces in response to plant physical conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to the following patent applications:

1. Ser. No. 399,790, filed on Sept. 21, 1973 by L. F. Martz, R. W.Kiscaden and R. Uram, entitled "An Improved Gas Turbine And SteamTurbine Combined Cycle Electric Power Generating Plant Having ACoordinated And Hybridized Control System And An Improved Factory BasedMethod For Making And Testing Combined Cycle And Other Power Plants AndControl Systems Therefor," assigned to the present assignee and herebyincorporated by reference.

2. Ser. No. 408,972, which is a continuation of Ser. No. 247,877, nowabandoned, which is a continuation-in-part of Ser. No. 247,440, nowabandoned, which is a continuation-in-part of Ser. No. 246,900, filedApr. 24, 1972, entitled "General System And Method For Starting,Synchronizing And Operating A Steam Turbine With Digital ComputerControl," all filed by T. C. Giras and R. Uram and assigned to thepresent assignee, and hereby incorporated by reference.

3. Ser. No. 495,722, filed concurrently herewith by J. Smith et al,entitled "Improved Digital/Analog Interface System, Especially Useful InTurbine And Power Plant Control Systems," assigned to the presentassignee.

4. Ser. No. 495,765, filed concurrently herewith by Lyle F. Martz andRichard J. Plotnick, entitled "Combined Cycle Electric Power PlantHaving A Control System Which Enables Dry Steam Generator OperationDuring Gas Turbine Operation," assigned to the present assignee andhereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to combined cycle electric power plants,and more particularly to improved steam turbine valve controls,particularly useful in the operation of a steam turbine having adigital/analog control system.

In the generation of electric power in steam turbine plants includingthe type described, the turbine inlet steam control valves and the steamturbine bypass valve are controlled automatically by a programmeddigital computer having an analog backup control. Certain of the valves,such as the main bypass and the control valves, for example, may beoperated sequentially; that is, one of the valves must reach a certainposition before the next one opens and closes; or the valves may beoperated singly; that is, a number of valves may be operated as one.

In conventional installations, the operating mechanism of the valves istypically controlled by an analog signal, the value of which determinesthe position of the valve. Also, in systems presently in use an outputfrom the digital computer provides an input to a resistive network whichconverts the digital signal to an analog signal.

When operating in either the digital or analog mode, it is desirable forthe system not in control to track the controlling signal so that thevalve positions remain unchanged when transferring from one to theother. This provides a so-called "bumpless" transfer. In such priorsystems, a separate tracking scheme is required when operating in theanalog mode for transfer to the digital mode. In order to simplify boththe analog and digital portions of the control system, it is desirableto have a similar input signal to the interface of the two systems,without the necessity of converting one type of interface input toanother.

In the analog portion of the control system, it is desirable to vary theduration of an input to the interface instead of the value of the input,because it can be controlled within very close limits or tolerances.Also, such analog system can use comparative techniques in order toproduce the required input to the interface. In the digital portion ofthe system, an input can occur during a required number of successivecycles to operate the valves to the controlled position. This simplifiesboth the operation of the two systems and the transfer from one of thesystems to another.

Further, it is desirable for the manual control to be able to overridethe digital control without the necessity of transferring from one modeto another.

In stand alone steam turbine power plants, which are often unattended, asimilar kind of digital/analog control can be desirable.

SUMMARY OF THE INVENTION

A combined cycle electric power plant includes a digital/analog controlsystem for operating the steam turbine valves. The digital systemgenerates repetitive output pulses in response to plant inputs. Theanalog system generates an output pulse the duration of which depends onplant inputs. A hybrid interface for each valve selectively responds tothe respective pulses and the output pulse of the respective digital andanalog systems to generate an analog representation, and value of whichdepends on the duration of the analog pulse or the digital pulses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a combined cycle electric powerplant in which there is employed a steam turbine in accordance with theprinciples of the invention;

FIGS. 2A, 2B and 2C when placed side-by-side show a detailed schematicdiagram of one embodiment of a combined cycle electric power plant inwhich the principles of the present invention are utilized;

FIG. 3 shows a schematic view of a control system arranged to operatethe plant of FIGS. 1, 2A, 2B and 2C in accordance with the principles ofthe invention;

FIG. 4 illustrates a steam turbine structure which can be employed inthe plant of FIGS. 1, 2A, 2B and 2C;

FIG. 5 is a graphical illustration of pressure versus flowcharacteristics of steam with one or both of the gas turbines operatingin conjunction with the steam turbine and illustrating the effect of thethrottle pressure limiting function of the control system of FIG. 3 forthe plant of FIGS. 2A, 2B and 2C;

FIGS. 6A, 6B and 6C when placed side-by-side show a functional blockdiagram of a throttle pressure limit control arranged in accordance withthe invention;

FIGS. 7A, 7B and 7C when placed side-by-side show a functional blockdiagram of a system for operating the steam turbine control valves inaccordance with the present invention;

FIG. 8 shows a schematic diagram of a circuit card which provides ahybrid interface between the digital computer circuitry and the analogcircuitry employed in the steam turbine control system;

FIGS. 9A, 9B and 9C when placed side-by-side show a functional diagramof a main bypass valve control for the steam turbine arranged inaccordance with the invention;

FIGS. 10A through 10L show when arranged according to FIG. 11 showcircuitry which can be employed to embody various functional blocks ofFIGS. 6A through 6C;

FIG. 11 shows the arrangement of FIGS. 10A through 10L;

FIGS. 12A through 12F when placed side-by-side show circuitry which canbe employed to embody various functional blocks in FIGS. 7A through 7C;

FIGS. 13A through 13F when arranged according to FIG. 14 show circuitrywhich can be employed to embody various functional blocks of FIGS. 9Athrough 9C;

FIG. 14 shows the arrangement of FIGS. 13A through 13F;

FIGS. 15A and 15B show a functional diagram of the digital controlsystem for the steam turbine;

FIGS. 16A and 16B is a flow chart of the steam turbine throttle pressuremonitor shown in a block form of FIG. 15;

FIG. 17 is a flow chart of the valve position limit shown in block formof FIG. 15;

FIG. 18 is a flow chart of the steam bypass valve control;

FIGS. 19A and 19B are flow charts of the steam turbine speed monitorshown in block form in FIG. 15;

FIG. 20 is a flow chart of the manual track shown in block form of FIG.15;

FIG. 21 is a flow chart of the steam turbine speed load control;

FIG. 22 is a flow chart of the steam turbine auto/manual logic.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A. General Plant Description

Referring to FIG. 1 of the drawings, there is shown a functional blockdiagram of a representative embodiment of a combined cycle electricpower generating plant constructed in accordance with the presentinvention. Reference numeral 10 is used to identify the combined cycleplant as a whole. As such, the plant 10 includes a first gas turbine 12(sometimes referred to as "gas turbine No. 1") which drives a firstelectric generator 13. Fuel is supplied to the gas turbine 12 by way ofa fuel control valve or throttle valve 14. Air enters the gas turbine 12by way of a variable inlet guide vane mechanism 15 which controls thedegree of opening of the turbine air intake and which is used to adjustair flow during the startup phase and to increase part load efficiency.The fuel supplied by the throttle valve 14 is burned in the gas turbine12 and the resulting high temperature exhaust gas is passed through anafterburner 16 and a heat recovery steam generator 18 and is thereafterexhausted into the atmosphere.

Heat recovery steam generator 18 (sometimes referred to as "heatrecovery steam generator No. 1") includes therein various sets of boilertubes which are heated to a relatively high temperature by the gasturbine exhaust gas passing through the steam generator 18. Afterburner16 includes a burner mechanism for further increasing the temperature ofthe gas turbine exhaust gas before it enters the steam generator 18.Fuel is supplied to the burner mechanism in the afterburner 16 by way ofa fuel control valve or throttle valve 19. The primary heat source forthe steam generator 18 is the gas turbine 12, the afterburner 16 beingin the nature of a supplemental heat source for providing supplementalheat when needed. In terms of typical fuel usage, approximately 80% ofthe fuel is used in the gas turbine 12 and 20% is used in theafterburner 16.

The combined cycle plant 10 further includes a second gas turbine 22(sometimes referred to as "gas turbine No. 2") which drives a secondelectric generator 23. Fuel is supplied to the gas turbine 22 by way ofa fuel control valve or throttle valve 24. Air enters the gas turbine 22by way of a variable inlet guide vane mechanism 25 which is used toadjust air flow during turbine startup and to increase part loadefficiency. The fuel supplied by throttle valve 24 is burned in the gasturbine 22 and the resulting high temperature exhaust gas is passedthrough an afterburner 26 and a heat recovery steam generator 28 and isthereafter exhausted into the atmosphere.

Heat recovery steam generator 28 (sometimes referred to as "heatrecovery steam generator No. 2") includes various sets of boiler tubeswhich are heated to a relatively high temperature by the gas turbineexhaust gas passing through the steam generator 28. Afterburner 26includes a burner mechanism for further increasing the temperature ofthe gas turbine exhaust gas before it enters the steam generator 28.Fuel is supplied to the burner mechanism in the afterburner 26 by way ofa fuel control valve or throttle valve 29. The primary heat source forthe steam generator 28 is the gas turbine 22, the afterburner 26 beingin the nature of a supplemental heat source for providing supplementalheating when needed. In terms of typical total fuel consumption,approximately 80% of the fuel is used in the gas turbine 22 and 20% isused in the afterburner 26.

A condensate pump 30 pumps water or condensate from a steam condenser 31to both of the steam generators 18 and 28, the condensate flowing to thefirst steam generator 18 by way of a condensate flow control valve 32and to the second steam generator 28 by way of a condensate flow controlvalve 33. Such condensate flows through the boiler tubes in each of thesteam generators 18 and 28 and is converted into superheated steam. Thesuperheated steam from both of the steam generators 18 and 28 issupplied by way of a common header or steam pipe 34 and a steam throttlevalve or control valve 35 to a steam turbine 36 for purposes of drivingsuch steam turbine 36. The steam from the first steam generator 18 flowsto the header 34 by way of a steam pipe 37, an isolation valve 38 and asteam pipe 39, while steam from the second steam generator 28 flows tothe header 34 by way of a steam pipe 40, an isolation valve 41 and asteam pipe 42.

The spent steam leaving steam turbine 36 is passed to the condenser 31wherein it is condensed or converted back into condensate. Suchcondensate is thereafter pumped back into the steam generators 18 and 28to make more steam. Steam turbine 36 drives a third electric generator44.

A steam bypass path is provided for use at appropriate times fordiverting desired amounts of steam around the steam turbine 36. Thissteam bypass path includes a steam turbine bypass valve 45 and adesuperheater 46, the output side of the latter being connected to thecondenser 31 by way of a pipe 47. A drain valve 48 is provided for thefirst steam generator 18, while a drain valve 49 is provided for thesecond steam generator 28.

The operation of the combined cycle electric power generator plant 10 iscontrolled by a control system 50, typical control signal lines 51 beingshown in a broken line manner. As will be seen, the control system 50offers a choice of four different control operating levels providingfour different degrees of automation. From highest to lowest in terms ofthe degree of automation, these control operating levels are: (1) plantcoordinated control; (2) operator automatic control; (3) operator analogcontrol; and (4) manual control. The control system 50 includes ananalog control system which is constructed to provide complete and safeoperation of the total plant 10 or any part thereof. The control system50 also includes a digital computer that provides a real-time digitalcontrol system that works in conjunction with the analog control systemat the higher two levels of control to coordinate and direct theoperation of the analog control system. Failure of the digital controlcomputer results in no loss of power generation because the analogcontrol system provides for complete operation of the plant 10.

When operating at the highest level of control, namely, at the plantcoordinated control level, the control system 50, among other things,automatically coordinates the settings of the fuel valves 14, 19, 24 and29, the inlet guide vanes 15 and 25 and the steam turbine throttle andbypass valves 35 and 45 to provide maximum plant efficiency under staticload conditions and optimum performance during dynamic or changing loadconditions.

The control system 50 also enables a coordinated automatic startup orshutdown of the plant 10 such that the plant 10 can be brought from ahot standby condition to a power generating condition or vice versa in aquick, efficient and completely automatic manner. For example, theentire plant 10 can be started and brought to full load from a hotstandby condition in approximately 60 minutes time by having the plantoperator simply dial in the desired load setting and push a master plantstart button.

As an indication of the flexibility and reliability of the powergenerating plant 10, it is noted that the plant 10 can be operated inany one of the following configurations: (1) using one steam turbine andtwo gas turbines; (2) using one steam turbine and one gas turbine; (3)using two gas turbines only; and (4) using one gas turbine only. Thesteam turbine 36 will, of course, not operate by itself, it beingnecessary to use at least one of the gas turbines 12 and 22 in order touse the steam turbine 36. In order to obtain the benefits of combinedcycle operation, it is, of course, necessary to use the steam turbine 36and at least one of the gas turbines 12 and 22. When one of the gasturbines, for example the gas turbine 12, is not being used or is shutdown for maintenance purposes, then its associated steam generator 18can be removed from the system by closing its condensate flow valve 32and its steam isolation valve 38. When, on the other hand, the steamturbine 36 is not being used or is shut down for maintenance purposes,the steam generated by the steam generators 18 and 28 can be bypassed tothe condenser 31 by way of steam bypass valve 45 and the desuperheater46. As an alternative, when the steam turbine 36 is not being used,either one or both of the steam generators 18 and 28 can be drained andvented by the appropriate setting of condensate valves 32 and 33, steamisolation valves 38 and 41 and drain valves 48 and 49. In other words,each of the steam generators 18 and 28 is constructed so that itsrespective gas turbine can be operated with the steam generator in a drycondition.

The combined cycle plant 10 affords a high degree of reliability in thatfailure of any one of the major apparatus components will not reducetotal plant power generation capacity by more than 50%. In this regardand by way of example only, a combined cycle plant 10 has been developedhaving a nominal maximum power generating capacity of 260 megawatts. Insuch plant, each of the gas turbines 12 and 22 is capable of producing amaximum of approximately 80 megawatts of electrical power under ISOconditions (59° Fahrenheit at sea level) and the steam turbine 36 iscapable of producing a maximum of approximately 100 megawatts ofelectrical power. Thus, loss of any one of the turbines 12, 22 and 36,for example, would not reduce total plant capacity by as much as 50%.

It is noted in passing that the functional block diagram of FIG. 1 hasbeen simplified in some respects relative to the actual plant apparatusto be described hereinafter, this simplification being made tofacilitate an initial overall understanding of the combined cycle plant10. A major simplification in FIG. 1 concerns the fuel valves 14, 19,24, and 29. As will be seen in the actual embodiment of the combinedcycle plant described herein, provision is made for operating the gasturbines 12 and 22 and the afterburners 16 and 26 on either of twodifferent kinds of fuel, namely, either natural gas or distillate typefuel oil. As a consequence, each of the gas turbines 12 and 22 and eachof the afterburners 16 and 26 is actually provided with two fuelthrottle valves, one for natural gas and the other for fuel oil. Also,various other valves and devices employed in the actual fuel supplysystems have been omitted from FIG. 1 for the sake of simplicity. Othersimplifications employed in FIG. 1 are of a similar character.

DETAILED PLANT DESCRIPTION

FIGS. 2A, 2B and 2C when placed side-by-side show in greater detail thevarious valves, pumps, measurement devices and other items associatedwith the operation of the steam turbine 36.

The same reference numerals used in FIG. 1 is used in FIGS. 2A, 2B and2C for elements previously described therein. In some cases, an itempreviously described as a single element will be described in FIGS. 2A,2B and 2C as two or more identical elements performing the samefunction, usually in parallel with one another. In such cases, the samereference numeral will be used but with suffix letters a, b, c, etc.added thereto to distinguish the different ones of the identicalmultiple elements.

Located at the lower end of the stack structure 64 of the first steamgenerator 18 a plurality of temperature elements, including temperatureelements 353 and 354, which provide indications of the steam generatorinlet gas temperature. Under typical peak load conditions, thistemperature will be on the order of approximately 1200° F as a result ofadded afterburner heat. A pressure switch 355 monitors the steamgenerator inlet gas pressure and produces a warning signal if suchpressure exceeds a desired limit. Located at the top of the stackstructure 64 is a further temperature element 356 which produces asignal indicative of the gas top temperature at the top of the stack.Under typical peak load conditions, this temperature will beapproximately 340° F. Thus, under typical peak load conditions, the gastemperature varies from about 1200° F at the bottom of the stack 64(temperature element 353) to about 340° F at the top of the stack 64(temperature element 356). The gas temperature intermediate thesuperheater tubes 260 and the high pressure evaporator tubes 261 isabout 1000° F. The gas temperature intermediate the high pressureevaporator tubes 261 and the economizer tubes 262 is about 600° F. Thegas temperature intermediate the economizer tubes 262 and the lowpressure evaporator tubes 263 is about 360° F.

Located at the top of the stack structure 64 is a stack cover louverstructure 357 which can be closed when the gas turbine 12 is not inservice. This stack cover mechanism 357 is operated by a motor 358. Inpassing, it is noted that there are actually two of these stack covermechanisms 357, one being located at the top of each of the two parallelstack structures 250 and 251 (FIG. 9).

Considering now the second heat recovery steam generator 28 (FIG. 2C),there is located within the stack structure 86 thereof superheater tubes360, high pressure evaporator tubes 361, economizer tubes 362 and lowpressure evaporator tubes 363. These tubes 360-363 correspond in purposeand function to the tubes 260-263, respectively, located in the stackstructure 64 of the first steam generator 18. Located at the lower endof the second stack structure 86 are turning vanes (not shown) whichturn the turbine exhaust gas upwardly through the tube sections 360-363.

Considering now the steam turbine 36, electric generator 44 andcondenser 31 (FIG. 2B) in greater instrumentation detail, a speedtransducer 401 is coupled to the rotary shaft structure of the generator44 and produces an electrical signal indicating the rotary speed or rpmof the rotor structures of the steam turbine 36 and generator 44. Undernormal load conditions, the steam turbine speed will be the synchronousvalue of 3,600 rpm and, during startup, the steam turbine speed willnormally be a controlled value as the turbine accelerates to synchronousspeed. A temperature element 402 and a pressure transmitter 403 generateelectrical signals which indicate the throttle temperature and thethrottle pressure of the steam entering the inlet of the steam turbine36. Under typical peak load conditions, the turbine inlet steamtemperature will be approximately 952° F and the turbine inlet steampressure will be approximately 1,277 pounds per square inch (absolute).The outlet side of the steam turbine bypass valve 45 is connected to thedesuperheater 46 by way of a steam pipe 404. A temperature element 405generates an electrical signal which indicates the temperature of anysteam flow from the desuperheater 46 to the condenser 31 by way of steampipe 47. Under typical load conditions with both of the gas turbines 12and 22 in operation, the bypass valve 45 is fully closed and no steamflows to the desuperheater 46.

Some of the steam in the incoming main steam pipe 34 is removed by wayof a steam pipe 406 and supplied by way of a check valve 407, asuperheater 408, a control valve 409 and a steam pipe 410 to the glandseals inside the steam turbine 36 to provide the desired sealing actiontherein. After passage through the gland seal structure, this glandsteam is removed by way of a pipe 411 and passed to a gland steamcondenser 412, the resulting condensate being passed to a drain tank(not shown) by way of a drain line 413.

Some of the steam in main steam line 34 is also supplied by way of acontrol valve 414 to an air ejector mechanism 415. Air ejector mechanism415 is a Venturi type air ejector which is used to evacuate thecondenser 31. The steam leaving the air ejector 415 passes by way of asteam line 416 to an air ejector steam condenser 417, the resultingcondensate being passed to the drain line 413. Air is removed from thecondenser 31 by way of a line 418 which runs to the air ejector 415. TheVenturi effect occurring in the air ejector 415 serves to suck the airout of the condenser 31 by way of the air line 418. Under typicaloperating conditions, this evacuates the condenser 31 to a pressure ofapproximately two inches of mercury.

Extraction steam for feedwater heating purposes is removed from thesteam turbine 36 between the tenth and eleventh stages thereof by way ofturbine outlet 313 and is supplied by way of a steam pipe 420, a checkvalve 421, a control valve 422 and steam pipe 423 to a pair of branchsteam pipes 424 and 425. The branch steam pipe 424 supplies extractionsteam to the deaerator 68 included in the first steam generator 18 whilethe branch steam pipe 425 supplies extraction steam to the deaerator 90included in the second steam generator 28. The "internal water removal"steam removed between the 12th and 13th stages via the steam turbineoutlet 314 is supplied by way of steam pipe 426 to the condenser 31.Level transmitters 428 and 429 produce electrical signals which indicatethe water levels in hotwell portions 335a and 335b, respectively.

Condensate is pumped from the two hotwell portions 335a and 335b of thedivided hotwell 335 by means of condensate pumps 30a and 30b,respectively. The inlet side of pump 30a is connected to the hotwellcondensate outlet 338a (FIG. 12), while the inlet side of pump 30b isconnected to the hotwell condensate outlet 338b (FIG. 12). Thecondensate pumped by pumps 30a and 30b is supplied by way of acondensate pipe 430, the air ejector steam condenser 417, the glandsteam condenser 412, a condensate pipe 431 and a condensate pipe 432 toa pair of branch condensate pipes 434 and 435. Branch condensate pipe434 runs to the deaerator 68 located in the first steam generator 18,while the second branch condensate pipe 435 runs to the deaerator 90located in the second steam generator 28. The condensate as it leavesthe pumps 30a and 30b is at a temperature of approximately 110° F. Thiscondensate flows through the coolant tubes in the air ejector condenser417 and the gland steam condenser 412 to provide the cooling actionwhich occurs in these condensers 417 and 412. A normally-open manualcrossover valve 436 is connected between the two hot-well outlets andcan be closed if half the condenser 31 is shut down for maintenancepurposes or the like. Each of the condensate pumps 30a and 30b hassufficient capacity to enable either pump alone to carry the full flowload in the event the other pump should fail.

Some of the condensate flowing in the pipe 431 is also supplied by wayof a pipe 437, a desuperheater control valve 438 and a pipe 439 to thedesuperheater 46. This condensate provides the cooling medium in thedesuperheater 46. The desuperheater 46 is of the water spray type suchthat the relatively cool condensate entering by way of pipe 439 issprayed into the relatively hot steam flow entering by way of the pipe404. Under typical conditions for such steam flow, this lowers the steamtemperature to about 350° F. The temperature signal produced by thetemperature element 405 coupled to the desuperheater outlet pipe 47 issupplied by way of a temperature transmitter (not shown) and atemperature controller (not shown) to the desuperheater control valve438 for purposes of regulating same to hold the temperature of thedesuperheater outlet steam in pipe 47 fairly constant.

If the condensate level in the hotwell portions 335a and 335b becomestoo low, than makeup water from a makeup water storage tank 440 issupplied to the hotwell portions 335a and 335b by means of a makeupwater pump 441, a makeup block valve 441a, a makeup control valve 442and a makeup water pipe 443 which runs to the makeup water inlet 340 onthe condenser 31. Conversely, if the condensate level in hotwells 335aand 335b becomes too high, then condensate is returned to the makeupwater storage tank 440 by way of a condensate return valve 444. In otherwords, the pump 441 is operated and the valves 442 and 444 are openedand closed as needed in order to hold the condensate level in hotwells335a and 335b fairly constant. This is accomplished by means of levelsensing switches (not shown) associated with the hotwells 335a and 335bwhich operate the appropriate control circuits (not shown) to controlthe pump 441 and the valves 442 and 444. Block valve 441a is fully openduring normal operation. When needed, additional water is supplied tothe makeup water storage tank 440 from an external water source by wayof a demineralizer 445 and a control valve 446.

A pair of auxiliary steam bypass lines 447 and 448 are connected by wayof a common bypass line 449 to the bypass steam pipe 404 which runs tothe desuperheater 46. Bypass line 447 enables steam from the first steamgenerator 18 to be passed directly to the desuperheater 46 under certainoperating conditions, while the bypass line 448 does likewise for thesecond steam generator 28.

Temperature elements 450 and pressure transmitters 451 generateelectrical signals which serve to monitor the temperature and pressureof the incoming circulating water. The circulating water leaves thecondenser 31 by way of the outlet pipe 110. A further temperatureelement 452 generates an electrical signal to monitor the temperature ofthe outgoing circulating water.

Various additional temperature elements, pressure transmitters, leveltransmitters and other measurement devices are associated with thecondenser 31 and the steam turbine 36, these items being omitted fromFIG. 2B for sake of simplicity.

Considering now the details of the first heat recovery steam generator18 (FIG. 2A), condensate from the condenser hotwells 335a and 335b issupplied to the deaerator 68 by way of the condensate pipe 434, a flowelement 501, the condensate control valve 32 (Cf. FIG. 1) and a checkvalve 502. A flow transmitter 503 cooperates with the flow element 501to provide an electrical signal which indicates the value of thecondensate flow rate through the flow element 501. Flow element 501provides a restriction in the flow path and flow transmitter 503measures the pressure difference across the restriction. As is wellknown, this pressure difference is indicative of the flow rate. Thus,flow element 501 and flow transmitter 503 constitute a well-known typeof flowmeter for measuring fluid flow.

Deaerator 68 provides a feedwater heating action as well as a deaeratingaction, and it is of the spray tray or jet tray type. The condensateentering from check valve 502 is sprayed by way of spray nozzles into atray structure which also receives steam from the low pressureevaporator tubes 263. More particularly, the water or condensatecollected in the deaerator 68 flows to a low pressure feedwater storagetank 69 which, among other things, serves as a storage reservoir for thedeaerator 68. Water from this storage tank 69 flows by way of a pipe504, a low pressure circulation pump 505, a standby electric heater 506,the low pressure evaporator tubes 263 and a pipe 507 to a steam inletinto the tray structure inside the deaerator 68. Low pressurecirculation pump 505 provides the desired fluid flow and the lowpressure evaporator tubes 263 in the stack structure 64 serve to convertthe water into steam. This steam is supplied to the deaerator 68 by wayof pipe 507 to heat the condensate entering the deaerator 68 from checkvalve 502. This provides a substantial portion of the desired feedwaterheating.

Electric heater 506 is used for standby heating purposes when the gasturbine 12 is not in service. If the gas turbine 12 is not in operationand if it is desired to maintain the steam generator 18 in a hot standbycondition, then the heater 506 is controlled by a temperature switch 508so as to maintain the temperature of the water in the storage tank 69 ata value of approximately 250° F. If, on the other hand, it is desiredthat the steam generator 18 be shut down for an extended period of timebut not drained, then electric heater 506 is used to provide freezeprotection. In this latter case, the heater 506 is controlled by atemperature switch 509 so as to prevent the temperature of the water inthis part of the system from falling below a value of 40° F. In both ofthese cases, the low pressure circulation pump 505 must be turned on andoperating. A pressure switch 510 monitors the operation of the pump 505and produces a warning signal if the pressure differential across thepump 505 becomes too low.

For total plant loads above approximately 80% of the total plantcapacity, supplemental feedwater heating is provided by the extractionsteam taken from the steam turbine 36. This extraction steam is suppliedto the deaerator 68 by way of the extraction steam pipe 424, a checkvalve 511, a motor operated isolation valve 512 and a steam pipe 513.The extraction steam control valve 422 is opened for plant loads abovethe 80% figure. Below this figure, the steam used for feedwater heatingis obtained from the low pressure evaporator tubes 263.

Deaerator 68 is provided with a low pressure vent valve 514 which iscontrolled by an actuator 515. Actuator 515 is of the solenoid type andis controlled by an appropriate control signal from the main operatorcontrol board in the plant control center building 150 (FIG. 3). Duringnormal operation, the vent valve 514 is kept fully open to allow air toescape from the deaerator 68. The deaerator 68 is also provided with apressure safety valve 516. Deaerator 68 is provided with a furtherpressure release mechanism which includes a check valve 517 and a dumpvalve 518, the outlet side of the latter being connected by way of apipe 519 to the auxiliary steam bypass pipe 447 by way of which steammay be returned to the desuperheater 46 and condenser 31. If thepressure within the deaerator 68 exceeds 160 pounds per square inch,dump valve 518 opens to dump steam in the deaerator 68 back to thecondenser 31. Among other things, this prevents a popping of the safetyvalve 516.

A pressure transmitter 520 senses the pressure within the deaerator 68and provides a signal indicative of the value thereof. Level switches521 and 522 monitor the water level within the low pressure storage tank69, switch 521 producing an electrical warning signal if the water levelis too high and switch 522 producing an electrical warning signal if thewater level is too low. A level transmitter 523 produces an electricalsignal indicative of the actual water level in the tank 69.

Boiler feedwater stored in the low pressure storage tank 69 is pumpedthrough the economizer tubes 262 in the stack structure 64 by means of amain boiler feed pump 524. The intake side of boiler feed pump 524 isconnected to the storage tank 69 by means of a feedwater pipe 525. Theoutlet side of boiler feed pump 524 is connected to the inlet side ofeconomizer tubes 262 by means of a check valve 526, a motor operatedblock valve 527 and a feedwater pipe 528. Valve 527 is open duringnormal operation. The electric motor which runs the boiler feed pump 524has a nominal rating of 1250 horsepower. A pressure safety valve 529 isconnected between the outlet side of pump 524 and the low pressurestorage tank 69. A pressure switch 530 monitors the pressure differenceacross the boiler feed pump 524 and produces an electrical warningsignal if such pressure difference falls below a desired lower limit. Afurther pressure switch 531 monitors the pressure in the feedwater pipe528 and produces an electrical warning signal if such pressure fallsbelow a desired lower level.

A standby boiler feed pump 532 is connected in parallel with the mainboiler feed pump 524 and the valves 526 and 527, the outlet side of thisstandby pump 532 being connected by way of a check valve 533 and amotor-operated block valve 534 to the feedwater pipe 528 which runs tothe inlet of the economizer tubes 262. During normal operation of thesteam generator 18, the standby pump 532 is turned off and the blockvalve 534 is closed. The electric motor which runs the standby pump 532has a nominal rating of 25 horsepower. The standby pump 532 is used whenthe steam generator 18 is in either the hot standby mode or the freezeprotection mode. At such time, the main boiler feed pump 524 is turnedoff and its block valve 527 is closed. A pressure safety valve 535 isconnected to the outlet side of the standby pump 532 and is connectedback to the low pressure storage tank 69. A pressure switch 536 monitorsthe pressure difference across the standby boiler feed pump 532 andproduces an electrical warning signal when the pressure difference istoo low.

A manually-operated drain valve 537 is provided for draining thedeaerator 68 and low pressure storage tank 69 when the steam generator18 is to be shut down for maintenance purposes or other desired reasons.A manually-operated vent valve 538 is connected to the economizerfeedwater pipe 528 for venting air from the system when the steamgenerator 18 is being shut down and the system filled with a nitrogenblanket. During normal operation, the drain valve 537 and the vent valve538 are closed.

During normal load operation, the main boiler feed pump 524 pumps boilerfeedwater through the economizer tubes 262, such feedwater beingobtained from the low pressure storage tank 69. Under typical peak loadconditions, the feedwater leaving the storage tank 69 will be at atemperature of approximately 250° F. As this feedwater flows through theeconomizer tubes 262, it is heated to within 5° F. of the saturationtemperature, that is, the temperature at which it will boil at thepressure at hand. Under typical peak load conditions, the feedwaterleaving the economizer tubes 262 will be at a temperature ofapproximately 570° F.

The hot feedwater leaving the economizer tubes 262 goes to two differentplaces. Firstly, some of this feedwater flows by way of a pipe 540, aflow element 541, a feedwater control valve 542, a check valve 543 and apipe 544 to the feedwater reservoir section 70a of the vertical steamdrum 70. The remainder of the hot feedwater leaving economizer tubes 262flows by way of pipe 540, a flow element 545, a recirculation controlvalve 546 and a pipe 547 back to the deaerator 68, wherein it serves toprovide some of the heating of the condensate entering the deaerator 68.

During normal load operation, the feedwater control valve 542 and therecirculation control valve 546 are automatically controlled in acoordinated manner to keep constant the water flow rate through theeconomizer tubes 262. For example, if less water is required by thefeedwater reservoir 70a (lower load level), then more water isrecirculated back by way of the valve 546 to the deaerator 68, theproportions being such as to hold constant the water flow in the pipe540. As the power generated by steam turbine 36 increases, moreeconomizer water flow is directed to the feedwater reservoir 70a.

Constant water flow through the economizer tubes 262 is important inorder to minimize steaming and prevent stagnation in some of theeconomizer tubes 262 at part loads. If the flow rate were not constantbut instead were allowed to vary with load, then the flow rate woulddecrease as the load decreased. At the lower flow rates, the likelihoodof steaming would be greater. The problem with steaming is that itproduces an increased pressure drop in the tube wherein it is occurring.This leads to less flow and more steaming and ultimately stagnation or acomplete absence of flow in such tube.

In the present embodiment, the flow rate is maintained constant at arelatively high value such that the same high water velocities areprovided in the various economizer tubes at all load levels. Thus, thepressure drop across the entire economizer section 262 is relativelyhigh at all load levels. Consequently, any increase in pressure dropcaused by steaming in certain tubes is small compared to the totalpressure drop, resulting in insignificant changes in water flow andthereby preventing stagnation in any of the economizer tubes 262. Inaddition, the higher pressure drops produced by the higher watervelocities through the economizer tubes 262 promotes a more uniformdistribution of water flow through the economizer tubes which, amongother things, results in higher heat transfer coefficients on the insideof the tubes.

A further advantage of the constant water flow rate through theeconomizer tubes 262 is that the main boiler feed pump 524 operates at aconstant and optimum rate in terms of pump efficiency for all plant loadlevels.

A flow transmitter 548 and a temperature element 549 are associated withthe feedwater flow element 541, with the flow transmitter 548 providingan electrical signal indicative of the feedwater flow rate through theflow element 541 and the temperature element 549 providing temperaturecompensation for the flow rate signal. Similarly, a flow transmitter 550and a temperature element 551 are associated with the recirculation pathflow element 545, with the flow transmitter 550 providing an electricalsignal indicative of the value of the flow rate of the water flowingback to the deaerator 68 and the temperature element 551 providingtemperature compensation for the flow rate signal. A manually-operateddrain valve 552 is connected to the feedwater pipe 540 for purposes ofdraining the economizer tubes 262 when the steam generator 18 is to beshut down. During normal operation, the drain valve 552 is closed.

The hot, nearly boiling feedwater in the feedwater reservoir 70a ispumped through the high pressure evaporator tubes 261 by a high pressurecirculation pump 554. The electric motor associated with this pump 554has a nominal rating of 60 horsepower. The inlet side of the pump 554 isconnected to the feedwater reservoir 70a by way of pipe 555. The outletside of pump 554 is connected to the high pressure evaporator tubes 261by way of a standby electric heater 556 and a pipe 557. As the hotfeedwater flows through the high pressure evaporator tubes 261 it isconverted into steam which is then supplied by way of a pipe 558 to themoisture separator section 70b of the steam drum 70. Under typical peakload conditions, the steam leaving the high pressure evaporator tubes261 will be at a temperature of approximately 575° F.

The high pressure circulation pump 554 is of a type which employsfloating ring type seals. The water required for these seals is obtainedfrom the economizer inlet pipe 528 by way of a pipe 560, awater-to-water heat exchanger or cooler 561, a motor-operated controlvalve 562 and a pipe 563. Heat exchanger 561 cools the 250° F. watercoming down from the feedwater pipe 528 to a temperature ofapproximately 150° F. The water leaving the pump seals is carried to adrain by way of a pipe 564.

With floating ring type seals, it is necessary to control the flow ofwater through the seals such that flashing will not occur since flashingof the water through the seals would result in erosion of the labyrinthelements in the seals. To this end, a temperature element 565 is locatedin the atmospheric collection chamber at the exit of the seals to sensethe temperature of the seal water leaving the seals. This temperatureelement 565 produces an electrical signal which is supplied to atemperature transmitter (not shown) which drives a temperaturecontroller (not shown) which, in turn, controls the motor 566 whichoperates the seal water control valve 562. This control loop modulatesthe control valve 562 to assure that only the required amount of wateris provided to the pump seals.

Water for the stuffing box in the high pressure circulation pump 554 issupplied thereto from the cooling water source for heat exchanger 561 byway of a pipe 567. The stuffing box water is drained by way of the drainpipe 564. A pressure switch 568 monitors the pressure difference acrossthe pump 554 and produces an electrical warning signal if this pressurebecomes too low. A further pressure switch 569 monitors the differentialpressure across the seals in pump 554 to provide an electrical warningsignal if this pressure differential becomes too low.

The electric heater 556 is used for standby and freeze protectionpurposes when the gas turbine 12 is not in service. When the gas turbine12 is not in operation and the steam generator 18 is in the hot standbymode, the heater 556 is controlled by a pressure switch 570 to maintainthe proper steam pressure in the steam drum 70. In other words, pressureswitch 570 turns on the heater 556 if the steam drum pressure fallsbelow the desired minimum value. On the other hand, if the plant 10 isshut down for an extended period of time and the plant operator choosesnot to generate steam in the steam generator 18, then the heater 556 iscontrolled by a temperature switch 571 to maintain the water in thesteam drum 70 above the freezing point. The high pressure circulationpump 554 must be kept on and operating during either of these operatingmodes for the heater 556.

The moisture separator section 70b of the steam drum 70 receives the wetsteam from the high pressure evaporator tubes 261 and removespractically all of the remaining water from such steam. The resultingdry steam leaves the moisture separator 70b and is supplied by way of asteam pipe 572 to the superheater tubes 260 located in the stackstructure 64. Under typical peak load conditions, the dry steam leavingthe moisture separator 70b is at a temperature of approximately 575° F.and a pressure of approximately 1300 pounds per square inch (absolute).

A pressure transmitter 573 generates an electrical signal whichindicates the steam pressure at the outlet of the moisture separator70b. A high pressure vent valve 574 is connected to the steam line 572for purposes of, among other things, venting some of the steam if itappears that the steam pressure inside the steam drum 70 is becoming toolarge. During normal operation, the vent valve 574 is closed. The steamdrum 70 is also provided with one or more pressure safety valves whichfor simplicity of illustration, are not shown.

A steam line 575 is connected from the main steam pipe 572 to adeaerator pressure control valve 576 which is, in turn, connected to anadditional steam inlet of the deaerator 68. The control valve 576 iscontrolled by a pressure controller 577 which is responsive to thepressure within the deaerator 68. Pressure controller 577 and controlvalve 576 function to maintain the desired steam pressure in thedeaerator 68 at part loads for the plant 10. If the steam pressurewithin the deaerator 68 falls below the desired value, then pressurecontroller 577 opens the valve 576 to bring the pressure back up to thedesired value. This is most likely to occur at part loads of less thanabout 80% because, in such cases, the extraction steam control valve 422(FIG. 2B) is closed and no extraction steam is being supplied to thedeaerator 68.

A nitrogen gas supply 578 is connected to the main steam pipe 572 by wayof an actuator-operated nitrogen admission valve 579 and a check valve580. During normal operation, the nitrogen admission valve 579 is closedand no nitrogen is admitted into the steam system. Valve 579 is openedduring the process of draining and venting the steam generator 18 andtransferring it to a dry status. The nitrogen valve 579 is opened asmore or less the final step in this process. The nitrogen gas isadmitted into the steam system for purposes of replacing steam whichcondenses in the system during the draining and venting process. Amongother things, this minimizes subsequent rusting or scaling in the steamdrum 70 and the evaporator and superheater tubes 261 and 260. Asmentioned elsewhere herein, the gas turbine 12 can be operated forprolonged periods of time with the steam generator 18 in a dryconditions without causing serious damage to the boiler tubes 260-263and other parts of the steam generator 18.

The feedwater reservoir section 70a of the steam drum 70 is providedwith a high-indicating level switch 581, a low-indicating level switch582 and a level transmitter 583. Switch 581 produces an electricalwarning signal when the water level in the reservoir 70a gets too high,while switch 582 produces an electrical warning signal when the waterlevel gets too low. Level transmitter 583 produces an electrical signalindicating the actual water level in the reservoir 70a. The water levelsignal from the transmitter 583 is supplied to a controller (not shown)which controls the feedwater control valve 542 to maintain a fairlyconstant water level in the feedwater reservoir 70a.

As the dry steam from the steam drum 70 flows through the superheatertubes 260, it is further heated to raise its temperature about 300° to400° F. Under typical peak load conditions, the superheated steamflowing in the main steam outlet line 37 is at a temperature of 952° Fand a pressure of approximately 1277 pounds per square inch (absolute).During normal operation of the plant 10, this superheated steam flows byway of main steam outlet line 37, isolation valve 38, steam pipe 34 andsteam turbine valves 35a, 35b, 308a and 308b to the main steam inlet ofthe steam turbine 36. Connected in series in the main steam outlet line37 are a flow element 584 and a check valve 585. A flow transmitter 586and a temperature element 587 (for temperature compensation of flowtransmitter 586) are associated with the flow element 584, the flowtransmitter 586 producing an electrical signal indicating the value ofthe output steam flow rate for the steam generator 18. During normalload operation, the main steam isolation valve 38 is, of course, fullyopen.

The final output steam temperature for the steam generator 18 is thetemperature of the superheated steam flowing in the steam generatoroutlet line 37. This temperature is primarily determined by thetemperature rise of the steam in the superheater tubes 260, thistemperature rise being dependent on the temperature of the exhaust gasleaving gas turbine 12 and the amount of supplemental heat added to theturbine exhaust gas by the afterburner 16. The final steam temperaturein outlet line 37 is also controlled in part by means of a superheaterbypass valve 588 which is connected between the inlet and outlet of thesuperheater tube section 260. More specifically, the outlet side ofbypass valve 588 is connected to the superheater outlet header 264 towhich is connected the steam generator outlet line 37.

Superheater bypass valve 588 controls the output steam temperature bybypassing some of the lower temperature steam coming from the steam drum70 around the superheater tubes 260 and then mixing this lowertemperature bypassed steam with the higher temperature superheated steamemerging from the superheater tubes 260. Other things being constant,the greater the degree of opening of the bypass valve 588, the greaterthe amount of the lower temperature steam which is bypassed and, hence,the lower the temperature of the steam flowing to the steam turbine 36.The maximum amount of steam that can be bypassed by the bypass valve 588is about 20% of the total steam flow from the steam drum 70.

The superheater bypass valve 588 is the final control element in atemperature control loop which is used to regulate the output steamtemperature to hold it fairly constant at a predetermined setpointvalue. In the present embodiment, this predetermined setpoint value is952° F. Also included in this temperature control loop is a temperatureelement 589 which senses the temperature of the steam flowing in theoutlet steam line 37 downstream of the bypass valve 588. Temperatureelement 589 cooperates with a temperature transmitter (not shown) toproduce an electrical signal which is transmitted to a temperaturecontroller (not shown) which controls the degree of opening of thesuperheater bypass valve 588. If the steam temperature in the outletline 37 is greater than the 952° F. setpoint value, then the temperaturecontroller sends a signal to the bypass valve 588 to increase the degreeof opening of such valve. This reduces the steam temperature in outletline 37 to bring it back to the 952° F. value. Conversely, if the steamtemperature in outlet line 37 is less than 952° F., the temperaturecontroller decreases the degree of opening of the bypass valve 588. Thiscauses more steam to pass through the superheater tubes 260 and thusincreases the temperature of the steam in the outlet line 37.

This type of temperature control system has several advantages. It issuperior to a system in which water is injected into the superheatedsteam to cool it because such a system could also send slugs of waterinto the steam turbine if its control valve failed. Since the presentsystem injects dry steam, this is not a problem. The present system isalso better than a system which controls steam temperature by varyingthe afterburner firing rate because it will respond more rapidly to loadchanges.

There is also connected to the main steam outlet line 37 anormally-closed manually-operated vent valve 590, a pressure safetyvalve 590a, a temperature element 591 and a pressure transmitter 592.During normal operation, the vent valve 590 and the previouslyconsidered drain valve 48 are closed. Temperature element 591 andpressure transmitter 592 generate electrical signals which indicate thetemperature and pressure of the steam in the outlet line 37 and transmitsuch signals to the plant control center building 150.

In certain situations, the main steam isolation valve 38 is closed andthe steam produced by the steam generator 18 is bypassed to thecondenser 31 by way of an auxiliary steam bypass path which includes acheck valve 593, a motor-operated block valve 594 and a steam line 595which runs to and connects with the auxiliary steam bypass line 447which communicates with the desuperheater 46 by way of pipes 449 and404. This particular arrangement wherein the main steam isolation valve38 is closed and the auxiliary bypass block valve 594 is open isemployed, for example, to drain the outlet steam line 37 of water whenthe No. 1 steam generator 18 is to be started up after the No. 2 steamgenerator 28 has already been put into operation and is busy supplyingsteam to the steam turbine 36.

The steam generator 18 further includes an automatic "blowdown"mechanism for minimizing the buildup of mineral deposits on the innerwalls of the high pressure evaporator tubes 261. This blowdown mechanismincludes a motor-controlled blowdown block valve 596 and a blowdowncontrol valve 597 which are connected in series between the feedwateroutlet pipe 555 of the steam drum 70 and an appropriate drain or seweroutlet 597a. During normal operation, the block valve 596 is full open.

The blowdown control valve 597 is controlled by a signal developed by aconductivity element 598 which continuously measures the conductivity ofa sample portion of the steam drum feedwater, which sample portion flowsby way of the block valve 596 and a cooler 599 to the drain outlet 597a.Conductivity element 598 is connected to the outlet side of the cooler599, the function of the cooler 599 being to cool the feedwater sampleto a temperature suitable for the conductivity element 598. Theconductivity element 598 cooperates with a conductivity transmitter (notshown) to generate an electrical signal indicative of conductivity,which signal is transmitted to a conductivity controller (not shown)which controls the blowdown control valve 597.

The conductivity element 598 provides an electrical signal whichindicates the electrical conductivity of the feedwater flowing in thesteam drum outlet pipe 555. The "hardness" or mineral content of thefeedwater in the steam drum outlet pipe 555 determines the conductivityof this feedwater. The greater the "hardness" or mineral content, thegreater the conductivity.

The conductivity element 598 and its associated conductivity controlleroperate to adjust the degree of opening of the blowdown control valve597 so as to keep the feedwater mineral content below a desired limit.If the feedwater mineral content increases above the desired limit, thenthe blowdown control valve 597 is opened to a greater degree to dump agreater amount of the steam drum feedwater into the drain outlet 597a.This tends to lower the water level in the system. This, in turn,signals the makeup water pump 441 and the makeup water valve 442 (FIG.2B) to add fresh demineralized water to the system. This brings themineral content of the water in the system back down to the desiredlevel.

As seen from the foregoing description, the heat recovery steamgenerator 18 includes not only the stack structure 64 and the variousboiler tubes 260-263 located therein, but also the deaerator 68, the lowpressure storage tank 69, the steam drum 70 and the various other items501-599 considered in connection therewith.

The normal operation of the heat recovery steam generator 18 will now bebriefly summarized for the case where the combined cycle plant 10 isoperating under typical peak load conditions. In this case, both of thegas turbines 12 and 22, both of the afterburners 16 and 26, both of theheat recovery steam generators 18 and 28 and the steam turbine 36 are inoperation. The condensate pumps 30a and 30b pump condensate at atemperature of approximately 110° F. from the condenser hotwell sections335a and 335b via pipes 430, 431, 432 and 434 to the deaerator 68wherein such condensate is deaerated and heated to a temperature ofapproximately 250° F. by heat from the steam from the low pressureevaporator tubes 263, the extraction steam from the steam turbine 36(via steam pipe 424) and the hot water being recirculated from theeconomizer tubes 262 by way of the recirculation control valve 546 andthe pipe 547. This heated 250° F. water is supplied to the low pressurestorage tank 69. At this point, the water is referred to as boilerfeedwater.

The boiler feedwater in the storage tank 69 is pumped through theeconomizer tubes 262 by the main boiler feed pump 524. As this feedwaterflows through the economizer tubes 262, heat from the turbine exhaustgas raises its temperature to within 5° F. of the saturationtemperature, that is, the temperature at which it will boil at theparticular pressure at hand. Typically, the hot feedwater leaving theeconomizer tubes 262 will be at a temperature of approximately 570° F.This hot feedwater flows to the feedwater reservoir 70a of the steamdrum 70, the water level in the reservoir 70a being controlled by thefeedwater control valve 542.

The hot feedwater in the reservoir 70a is pumped through the highpressure evaporator tubes 261 by the high pressure circulation pump 554.As the feedwater flows through the high pressure evaporator tubes 261,more heat from the turbine exhaust gas converts it into steam having atemperature of approximately 575° F. This steam is supplied to themoisture separator 70b which serves to remove practically all of theremaining moisture from such steam.

The resulting dry steam leaving moisture separator 70b flows by way ofsteam pipe 572 to the superheater tubes 260. As this steam flows throughthe superheater tubes 260, heat from the turbine exhaust gas at the gasentry end of the stack structure 64 raises its temperature to a value ofapproximately 952° F. The resulting superheated steam leavingsuperheater tubes 260 flows by way of steam generator outlet line 37 andsteam pipes 39 and 34 to the steam turbine 36, wherein it is used todrive the rotor blades of the steam turbine 36. At the same time, thesecond steam generator 28 is similarly making superheated steam which isalso flowing to the steam turbine 36 by way of steam pipes 42 and 34,this steam combining with the steam from the first steam generator 18 toproduce the total driving force for the steam turbine 36.

As will be considered in greater detail hereinafter, when the combinedcycle plant 10 is operative above a minimum load level with both steamgenerators 18 and 28 in operation, the steam turbine 36 is operated in apure turbine following mode. In this mode, the steam turbine bypassvalve 45 is fully closed and the steam turbine governor or controlvalves 35a and 35b and throttle or stop valves 308a and 308b are allfully open. In this case, the power developed by the steam turbine 36 isdetermined entirely by the steam generated by the steam generators 18and 28 which is, in turn, determined by the operating levels of the gasturbines 12 and 22 and the afterburners 16 and 26.

The hot gas produced by the gas turbine 12 and the afterburner 16 flowsvertically upward in the stack structure 64. On the other hand, thefluid in the superheater tubes 260 and the economizer tubes 262 flows ina downward direction, counter to the direction of gas flow. Thisdownflow or counterflow in the superheater and economizer sections 260and 262 provides better heat transfer for the steam and water movingthrough these sections. In the evaporator sections, namely, the highpressure evaporator 261 and the low pressure evaporator 263, the waterand steam flow is in the upward direction which is the same direction asthat of the hot gas flow. This is of particular importance with respectto the high pressure evaporator 261. Since the process of evaporation isisothermal, the temperature advantage is the same for either an upflowor a downflow design. The upflow design used for the high pressureevaporator section 261 is, however, more advantageous in that it permitsoperation at part loads by means of natural circulation should there bea failure of the high pressure circulation pump 554.

Considering now the No. 2 heat recovery steam generator 28, it is notedthat this steam generator 28 includes, in addition to the elementspreviously considered, various elements bearing reference numerals 601through 699, inclusive. These elements 601-699 are the same as elements501-599, respectively, previously considered for the first steamgenerator 18. These elements 601-699 serve the same purposes andfunction in the same manner as do the corresponding ones of counterpartelements 501-599 in the first steam generator 18. Thus, the second steamgenerator 28 is of the same construction as and operates in the samemanner as does the first steam generator 18. For this reason, a detaileddescription of the second steam generator 28 will not be given herein.

STEAM TURBINE MECHANICAL STRUCTURE

Referring now to FIG. 4, there is shown a longitudinal, partiallycross-sectional, elevational view of the steam turbine 36. Steam turbine36 is a 13-stage single-cylinder or single-element non-reheat type ofsteam turbine constructed for operation at a rated speed of 3,600 rpmand is capable of driving an electric generator for producing in excessof 107 megawatts of electrical power. The steam turbine 36 includes anouter casing 300 and an inner rotor structure 301 having a rotor shaft302 which is supported at the high pressure end of the turbine by abearing 303 and at the exhaust end of the turbine by a bearing 304. Theload being driven, in this case, the electric generator 44, is coupledto the high pressure end of the shaft 302. Thirteen sets of rotor blades305 are mounted on the rotor structure 301, while thirteen interveningsets of stationary blades 306 are supported by the casing 300.

Mounted on top of the casing 300, at the high-pressure end thereof, isan upper steam inlet valve assembly 307a which includes a steam stopvalve 308a followed by a steam control valve 35a. Attached to the bottomof the casing 300 is a second steam inlet valve assembly 307b whichincludes a steam stop valve 308b followed by a steam control valve 35b.For the sake of cross-reference, control valves 35a and 35b correspondto the control valve 35 shown in FIG. 1. Steam enters by way of the stopvalves 308a and 308b (which are open during normal turbine operation),passes through control valves 35a and 35b and feeds into a 360° F steaminlet or steam admission chamber 310 in the casing 300. In other words,there is a 100% arc of steam admission and both of the control valves35a and 35b communicate with this arc. During normal operation, both ofthe valve assemblies 307a and 307b are operated in unison to functionlike a single valve assembly. When desired, either of the valveassemblies 307a and 307b may be tested while the steam turbine 36 is inoperation.

The greater bulk of the steam passes through the various sets of rotaryand stationary blades 305 and 306 and leaves the steam turbine 36 by wayof an exhaust structure 312, the outer end of which is coupled to theduct 104 (FIG. 3) leading to the condenser 31. Some of the steam isextracted from the turbine 36 between the tenth and eleventh stagesthereof by way of an extraction steam outlet 313. As will be seen, thisextraction steam is supplied to the deaerators 68 and 90 (FIG 2B)associated with the steam generators 18 and 28 for providing some of thefeedwater heating performed in such deaerators 68 and 90. Thisextraction steam feedwater heating is typically employed for plant loadsof 80% or more. A portion of the steam is also removed between thetwelfth and thirteenth stages by way of outlet 314 and passed directlyto the condenser 31. This so-called "internal water removal" provides aturbine end loading which is less than the maximum allowable.

The steam turbine 36 is constructed to utilize incoming steam having apressure of approximately 1200 pounds per square inch and a temperatureof approximately 950° F. The height of the rotor blades 305 in the lastrow or set at the exhaust end of the turbine 36 is 28.5 inches, thisbeing a measure of the flow capacity of the steam turbine 36. The steamturbine 36 is capable of driving an electric generator to produce inexcess of 107 megawatts of electrical power.

PLANT CONTROL SYSTEM

The plant control system 50 is organized to operate the plant equipmentsafely through startup and loading with high reliability so that theplant is highly and quickly available to meet power demanded from it. Toachieve this purpose, the plant control system is preferably embodied indigital/analog hybrid form, and the digital/analog interface ispreferably disposed in a way that plant protection and plantavailability are enhanced.

Generally, the total plant power is controlled by controlling theoperating level of the turbines and the afterburners, but the steamturbine goes into a follow mode of operation once the steam bypassvalves are closed and the steam turbine inlet valves are fully opened.In the follow mode, the steam turbine produces power at a leveldependent on the steam conditions generated by the heat inputs to thesteam generators.

As shown in FIG. 3, the control system 50 includes a digital controlcomputer 58G, a digital monitor computer 100C and various analogcontrols for operating the plant equipment in response to processsensors 101C while achieving the described objectives. In this instancean automatic startup control for the steam turbine 36 is largelyembodied in the monitor computer 100C. An operator panel 102C providesnumerous pushbutton switches and various displays which make it possiblefor the plant to be operated by a single person. The pushbutton switchesprovide for numerous operator control actions including plant andturbine mode selections and setpoint selections.

In the operator analog or manual mode of operation, the operator setsthe fuel level for the gas turbines 12 and 22 and the afterburners 16and 26 through gas turbine controls 104C and 106C during loading, but ananalog startup control included in each of the gas turbine controls 104Cand 106C automatically schedules fuel during gas turbine startups. Inaddition, sequencers 108C start and stop auxiliary equipment associatedwith the gas turbines during gas turbine startups. The turbine bypasssteam flow and the turbine inlet steam flow are controlled by operatorvalve positioning implemented by a steam turbine control 110C duringsteam turbine startup and loading in the operator analog mode. Certainautomatic control functions are performed for the steam and gas turbinesby the controls 104C, 106C and 110C in the operator analog mode.

In the operator automatic mode, the computers 58G and 100C performvarious control functions which provide for automatic startup andautomatic loading of the gas and steam turbines under the direction ofthe operator on a turbine-by-turbine basis. Afterburner controls 112Cand 114C and boiler controls 116C and 118C operate under operatorsetpoint control during the operator analog and operator automaticmodes. Respective digital/analog hybrid circuits 120C, 122C and 124Cinterface the digital and analog controls.

Under plant coordinated control, the computer 58G generally directs theplant operation through startup, synchronization and loading to producethe plant power demand. The extent of coordinated plant control isdependent on the existing plant configuration, i.e. according to theavailability of apparatus for operation or for coordinated operation.For example, if a gas turbine is shut down, it is excluded fromcoordination. Similarly, if the gas turbine has been excluded fromcoordinated control by the operator, the computer 58G will operateaccordingly. In all coordinated control cases, the boiler controls 116Cand 118C function separately, i.e. they react automatically to operatorsetpoints and signals generated by the process sensors 101C to controlthe steam generators according to plant conditions produced bycoordinated turbine and afterburner operations. The computer 58Gprovides setpoint signals for the afterburners in the coordinatedcontrol mode but not in the operator automatic mode. Coordinated controlprovides the highest available level of plant automation, and theoperator automatic and operator analog modes provide progressively lessautomation. Some parts of the analog controls function in all of theplant modes.

Generator synchronization is performed by a synchronizer 126C underoperator control or under computer control in the coordinated mode.Generally, the respective generators are sequenced throughsynchronization by switching actions applied to the synchronizer inputsand outputs.

Once the plant reaches hot standby and either gas turbine or both gasturbines have been started, the steam turbine can be started whenminimum steam supply conditions have been reached. Thereafter, theturbines are accelerated to synchronous speed, the generators aresynchronized and the fuel and steam valves are positioned to operate theturbines at the demand load levels. The manner in which the controlsystem 50 is configured and the manner in which it functions throughoutstartup and loading depends on the selected plant mode and the selectedor forced plant configuration and the real time process behavior.

DIGITAL/ANALOG HYBRID COUPLER (NHC) CARD

NHC card 801 (FIG. 8) converts a 12 bit binary number from the computerto an analog output signal. This card operates in either the manual orthe automatic mode. In the automatic mode, the NHC card output can beset or read by a computer peripheral channel. In the manual mode, theNHC card output is controlled by signals generated outside the computerwhich raise or lower the output.

In automatic operation, if the computer does not update the NHC cardwithin a set time period, the card is set to the manual mode by an alivecircuit. The alive circuit has a timing device which can be set for 1,5, or 20 seconds. The time period is selectable by resistor andcapacitor values.

In manual operation, clock pulses determine the rate of change of theanalog output signal. The clock pulses may be generated by either anexternal or an internal clock.

Automatic Operation

The computer uses a 14 bit word to send and receive data and status.When the address recognition circuit senses that the computer isaddressing the NHC card, it gates the data and status bits through theoutput gates. The status bits are routed to the register control and thedata bits are routed to the up/down counter. The status bits are decodedand appropriate action is taken. The output of the up/down counter(which contains the last word from the computer) is converted to a pulsetrain by the digital/pulse converter. The pulse train is then convertedto an analog signal. The output of the up/down counter and the statusbits are routed to the input gates and sent to the computer.

Manual Operation

In the manual mode, the count in the up/down counter is regulated byexternal raise (RPBIDL) and lower (LPBIDL) signals generated either bypushbuttons from a manual/automatic control station or by logiccircuitry. The clock will increment or decrement the counter as long asthe raise or lower signal is present. Roll over is inhibited; that is,the up/down counter cannot count past 4095 or below 0. The clock rate,which is adjustable by analog control, i.e. by means of a variablevoltage at pins 4 and 5, determines the amount of time it takes tochange the signal level. When the raise or lower signal goes low, i.e.logical zero, the count in the up/down counter is held; thus, the analogoutput signal remains constant at that level. The D/A register consistsof a set of binary up/down counters which accept parallel data and actas latches in the Automatic mode. In Manual mode the operator (orexternal logic) has control of the counters and can count them up ordown. The raise/lower logic and the clock control this process. Theraise and lower inputs control which direction the counters move. Thecounting rate is determined by the clock. If both raise and lower areenabled simultaneously the counters will do nothing.

Manual/Auto Transfer

If forced to Manual mode, the analog output signal remains unchanged atits last value until increased or decreased manually; thus, the transferis bumpless. The external interrupt alerts the computer to a change inthe card's operating mode. It is activated when the card goes from Autoto Manual or from Manual to Auto for any reason. A manual to autotransfer may be initiated only by the operator depressing the Autopushbutton. The card will remain in Manual mode if any internal orexternal "Go To Manual" signal exists. A "Ready" output indicates thatthe card is in Manual mode and that no "Go To Manual" signal is present.The card can be forced to Manual by a "Go To Manual" signal. An internal"Go To Manual" is generated by the computer outputting a "Go To Manual"status, by either a "Raise" or "Lower" input, or by the Keep Alivecircuit. After an Auto to Manual transition, the last number set in theD/A register by the computer remains until changed by the operator.

Referring to FIG. 5, line BF represents the normal pressure flowrelationship with steam being generated by both HRSG's; and with thesteam turbine control valves fully open. This rate of steam generationfor operation of the steam turbine in one actual installation does notrequire throttling on the control valves, except to maintain a minimumpressure of in the order of 300 pounds per square inch as represented byline AB in order to satisfy the requirements of the heat recovery steamgenerators HRSG. Thus, the pressure can slide from 300 to a maximum of1200 pounds depending on the steam flow or load of the turbine. The rateof steam generation for both HRSG's provides sufficient steam densitywith the pressure flow relationship being such that the probability ofwater carryover into the turbine and the erosion of the steam generatortubes is at a minimum.

For one HRSG operation, the normal pressure flow relationship with thesteam control valves wide open is represented by line OCD. The minimumpressure requirement of the HRSG necessitates throttling of the controlvalves in the zero to approximately 48% flow range as represented byline AC. The rate of steam generation and thus density of the steam inthe 70 to 100% flow range with the steam control valve wide open asrepresented by line CD provides a pressure flow relationship wherein thevelocity increases the probability of water carryover into the steamturbine and such steam velocity could also accelerate tube erosion.

Thus, for a rate of steam generation corresponding to one HRSG inservice, the maximum permissible velocity requires throttling of thesteam control valves above approximately 70% flow to provide a pressureflow relationship as represented by line GE. Thus, with one HRSG inoperation, the pressure can slide along line GE for the correspondingflow or load, and the system should be controlled so that such pressureflow relationship will not fall below line GE.

In the present embodiment of the invention, the throttle pressurelimiting control is implemented both in analog hardware and a programmeddigital computer. The programmed digital computer automatically controlsthe turbine control valves to permit the pressure to slide betweenapproximately 25% load and 100% load along and above line BF withoutthrottling when both HRSG's are in service; and modulates the controlvalves to permit the pressure to slide above and along line GE asrepresented by dashed line 601 when only one HRSG is in service. It alsomaintains minimum pressure. The analog system throttles the controlvalves to prevent the pressure flow relationships from falling belowline GE, when one HRSG is in operation; and maintains minimum pressurewhen one or both of the HRSG's are in operation. It also tracks andstores the actual operating pressure.

Another operational situation, which can create excessive steam velocityand affect the shrink-swell characteristics of the steam generation,thus increasing the probability of water carryover into the turbine, isthe transfer from two HRSG operation to one HRSG operation. In thisevent, the pressure flow relationship can be maintained above the lineGE by both the digital and analog control system, but the rapid decay ofpressure without throttling between lines BF and GE, particularly in thehigh flow range can increase such probability. Therefore, the analogsystem includes a feature for limiting the rate of pressure decay untilthe pressure is on the line GE. Without such additional feature, thesteam pressure would decay to line GE in the order of a fraction of asecond, while with such feature, the rate of decay is appreciablylonger.

Referring to FIGS. 6A, 6B and 6C, the throttle pressure limiting systemis controlled under various conditions of operation by the throttlepressure in the main steam header, the flow in each steam generatorHRSG, and the closed condition of the main bypass valve. The throttlepressure is sensed in the line 34 (see FIG. 2B) and then an appropriatesignal is transmitted by pressure transmitter 403. A signalrepresentative of flow from the steam generator HRSG No. 1 istransmitted via the flow transmitter 586 (see FIG. 2A); and a signalrepresentative of flow from steam generator HRSG No. 2 is transmittedvia the flow transmitter 686 (see FIG. 2C).

Referring to hardwired system of FIGS. 6A, 6B and 6C, and moreparticularly to FIG. 6A, the throttle pressure signal from thetransmitter 403 is applied at an input 701'. The superheated steam flowfrom the transmitter 586 is applied at an input 702', and thesuperheated steam flow from the transmitter 686 is applied at an input703'. Additionally, a switch 704' senses the open or closed condition ofthe main steam bypass valve.

Inputs 705' and 706', to which is applied a signal representative of theoutlet pressure from the steam generators HRSG1 and HRSG2, respectively,is utilized only in the event of a throttle pressure signal failure.Inputs 707' and 708', to which is applied a signal representative of athreshold level of flow above 30 and 60% of maximum respectively, isutilized in the event of a signal failure on input 702' or 703'. Theportion of the circuitry of FIG. 6A, involved with providing operabilityin the event of a signal failure such as may be caused by a transducerfailure, for example, is evident from the drawings; and the hardwiredthrottle pressure limiting system will be described for the sake ofsimplicity under non-failure input conditions in response to the inputs701', 702', 703' and 704'.

The throttle pressure limiting system in response to the inputs701'-704', broadly performs three primary functions; namely, itdetermines the number of steam generators that are in operation, selectsthe pressure flow relationship for the steam turbine in accordance withsuch determination, and either tracks the throttle steam pressure, orrunbacks the steam control valve to maintain the selected pressure flowrelationship as described in connection with FIG. 5.

The flow signal on input 702' is applied to a function block 711', whichprovides a digital signal on output 712' when the superheat outlet steamflow from HRSG No. 1 is greater than 30% of its maximum. This output on712' is applied to an AND gate 713', which provides a digital outputwhen there is a no failure condition on its other input 714'. The ANDgate 713' through OR gate 715' provides a digital output to an AND gate716'. In addition to the flow from HRSG No. 1 being greater than 30%,which eventually provides one of the digital inputs to the AND gate716', inputs 717', 718' and 720' must have a digital signal present inorder for the AND gate 716' to provide a digital output on a linereferred to as 721' to indicate that HRSG No. 1 is in service. The input717' is satisfied when it is indicated that the generator HRSG No. 1 isnot tripped; the input 718' is satisfied when it is indicated that themain steam block valve for generator HRSG No. 1 is not closed, and theinput 720' is satisfied when it is indicated from the input 704' thatthe main steam bypass valve is closed. The digital output on line 721'from the AND gate 716' is conducted to an exclusive OR function block722', which provides a digital output when only one of its inputs 721'or 735' has a digital signal applied thereto. The output on line 721' toindicate that HRSG No. 1 is in service is connected to a transferfunctional block or switch 723' to pass the signal from input 702'.

Similarly, a flow signal on input 703' for HRSG No. 2 is applied to afunction block 724', which provides a digital signal on its output 725'when the superheated outlet steam flow from HRSG No. 2 is in excess of30% of its maximum. The output on line 725' is applied to an AND gate726' which in turn is applied to an OR gate 727'. The AND gate 726' hasanother input 728' which indicates a no fail condition on input 703'.Thus, when the outlet steam flow exceeds 30% for HRSG No. 2, a digitalsignal is present on output 730' which is applied to an AND gate 731'.The AND gate 731' also includes an input 732', 733' and 734', all ofwhich must have a signal present in order for the AND gate 731' toprovide a distinctive signal on its output 735' which leads to theexclusive OR gate 722'. A signal on the input 732' indicates that thegenerator HRSG No. 2 is not tripped, a signal on the input 733'indicates that the main steam block valve for HRSG No. 2 is open, and asignal on the input 734' indicates that the main steam bypass valve isclosed. As previously mentioned, the exclusive OR function 722' whichhas an input 721' and 735' from generator HRSG No. 1 and HRSG No. 2respectively provides a distinctive output when only one or the other ofthese two inputs has a distinctive signal. Thus, in order for theexclusive OR gate 722' to provide a distinctive signal on its output737', one or other of the generators must be out of service. The output735' is also connected to a switch 738', which passes the flow signalfrom the input 703'.

The switch 723' conducts an analog signal representative of thesuperheat outlet steam flow for generator HRSG No. 1 to an input 740' ofa summing device 741' and the switch 738' conducts a signal on input739' of the summing device 741'. Thus, when a digital signal is presenton the input 721' or 735' indicating that HRSG No. 1 or HRSG No. 2,respectively is in service, an analog signal representative of theoutlet steam flow is present on the respective input 740' or 739' aslong as there is no failure condition. Should there be a failurecondition, switch block 742' or 743', as the case may be provides asignal corresponding to maximum flow of HRSG No. 1 or HRSG No. 2. Thedevice 741' which compares the values of the flow signal from inputs702' and 703', passes the largest of the signals to a function generator744'. The function generator 744' produces an output functioncorresponding to a predetermined pressure flow characterizationcorresponding to the line GE of FIG. 5.

The output from the function generator 744' is conducted to a transferfunction block 745', which determines whether the signal from thefunction generator 744' or a signal from transfer function block 746' isconducted to a summing device 747' over its output 748'.

When only one HRSG is in service, as indicated by a digital signal onoutput 737' from the exclusive OR gate 722', the output of the functiongenerator 744' is conducted to the summing device 747'. When both HRSG'sare in service, there is no signal present on the output 737'; and theswitch 745' permits the actual throttle pressure applied at input 701'to be applied to the summing device 747' through switch 746' output 750'indicating a no failure condition. Thus, when both HRSG's are inservice, an analog signal representative of the actual throttle pressureis applied to the summing device 747'; and when only one HRSG is inservice, an analog signal from the output of the function generator 744'is applied to the summing device 747' over the input 748'.

The device 747' directly compares the analog value of the signal of onepolarity on the line 748' with a signal of opposite polarity on theinput 751', which is connected through an amplifier 749' to anintegrator 752'. When the analog signal on the line 751' is greater thanthe analog signal on the line 748', a digital output occurs on outputline 753' of the comparing device 747' and when an analog signal oninput line 748' is greater than the analog signal on the line 751', adigital output occurs on the output 754' of the device 747'.

A digital signal on the output 754' causes a switch or transfer functionblock 755' to pass a first analog setpoint signal represented by afunction block 756', to a summing device 757' by way of an input 758'. Adigital signal on the output 753' causes a transfer function block 760'to pass a second analog setpoint signal represented by block 761' to thesumming device 757' via an input 762'. The function block 756'represents a setpoint signal of suitable value to increase the signal atthe output of the summing device 757'; and the function block 761'represents a signal of suitable value to decrease the signal at theoutput of the summing device 757'. The output of the summing device 757'is connected to an amplifier 763'. Thus, when the input signal 751' isgreater than the input signal 748', the input to the amplifier 763' isdecreased; and when the input signal 748' is greater than the inputsignal 751' the signal to the amplifier 763' is increased. When theanalog signals on the inputs 751' and 748' are equal, there is noeffective output from the summing device 757' to the amplifier 763'. Theamplified analog signal from the amplifier 763' is applied to anintegrator 752' which serves as a memory device. The output of theintegrator 752' is applied through block 749' to provide an equal andopposite signal for the other input 751' of the comparator 747' aspreviously mentioned. The output of the integrator 752' is also appliedto a switch 764' which passes the output of the integrator 752' to acomparator 765' only when one HRSG is in service. When both HRSG's arein service, switch 764' blocks the integrated signal from the integrator752' and such integrator tracks the value of the analog signal on theinput 737' or 750' depending on whether one or both of the HRSG's are inservice. The actual flow is tracked while both HRSG's are in operationto provide for the transfer from a two HRSG operation to a one HRSGoperation.

The comparator 765' provides an output on line TPL only when the analoginput signal on input line 768' from the low limit device 769' isgreater than the signal on input line 767' representing the actualthrottle pressure from the input 701'. The output on line TPL operatesthe steam inlet control valves toward a closed condition as describedlater herein.

The integrated signal from the integrator 752' is blocked by the switchor transfer function block 764' when both HRSG's are in service, and thelimiting function 769' provides a pressure reference output for thesumming device 765', corresponding to a minimum 300 pound throttlepressure for the steam generator. Thus, during a two generator operationthe actual throttle pressure on input 767' to the summing device 765' iscompared to the pressure reference on input 768' from the low limitdevice 766'. Under this condition of operation. Output TPL does notoccur to operate the control valves of the steam turbine towards aclosed position until the actual throttle pressure is less than thepressure reference or 300 pounds.

In the event the system transfers from a two HRSG operation to a oneHRSG operation, in response to a turbine trip, for example, theexclusive OR gate 722' provides a digital output signal on the line 737'which operates the transfer function block 745' to block the actualthrottle pressure analog signal on line 750', and pass the analog signalrepresentative of a characteristic pressure flow relationship from thefunction generator 744'. As previously mentioned, the input to suchfunction generator 744' is an analog signal representing the outletsteam flow from that generator HRSG which is still in service fromeither input 703' or 702' applied to the summing device 741'.Simultaneously, the transfer function 764' is switched in response to adigital signal on its input 737' to pass the integrated signal from theintegrator 752' through the limit function 769' to the comparator 765'.In such event, and assuming that both generators HRSG are operating inthe 100% flow range just prior to the transfer to a one HRSG operation,the integrated signal on 768' to the summing device 765' becomes greaterthan the actual pressure on input 767' which starts to close the valve.Also, the analog input to the summing device 747', which is now thecharacteristic pressure flow relationship, causes the signal on input748' to change depending on the flow from the HRSG in service to operatethe valve to a closed position depending on actual pressure as thepressure flow relationship decreases below the line GE as shown on FIG.5. As long as such pressure flow relationship is above the line GE, thenthe input 748' remains in a static condition and the inputs to thecomparator 757' are equal and there is no output on either 753' or 754',and thus, no analog output on TPL to operate the valves to a closedposition.

During this transfer operation, the gain of the amplifier 763' ischanged in accordance with the presence of a digital signal on input773', 775' or 766'. A change in the condition of the output 737' inresponse to a transfer from a two generator operation to a one generatoroperation also produces a signal on input 770' to a one shotmulti-vibrator 771'. The operation of the one shot multi-vibrator sets aflip-flop or function block 772' to produce a digital signal on itsoutput 773' connected to the amplifier 763' to change the rate of gainfrom the fast tracking rate to a slow transfer rate. The integrator 752'remembers the initial pressure prior to changing the operation from atwo HRSG to a one HRSG state, and the pressure reference to thecomparator 765' on input 768' is permitted to change slowly inaccordance with the slow rate of gain of the amplifier 763'. During thetime that the flip-flop 772' is in a state to provide a digital signalon the line 773', the transfer functon 760' is switched to decrease theoutput of the integrator 752' in accordance with the setpoint from thefunction 761'. As long as the input from the integrator 752' is greaterthan the actual throttle pressure on the line 767', output will occur onthe line TPL, and the control valves to the turbine will continue toclose gradually in accordance with the selected rate of gain. Inasmuchas the gain of the amplifier 763' is at a slow rate, during suchtransfer while the flip-flop 772' has an output on the input 773', thepressure reference on the input 768' changes very slowly until the oneshot multivibrator 771' resets the flip-flop 772' to provide an outputto AND gate 774' for providing a digital signal on input 775' to providea medium rate of gain for the amplifier 763'.

When the outputs 753' and 754' of the comparator 747' are equal,indicating that the pressure flow relationship is on the line GE, thereis no potential difference across inputs 776' and 777' of an OR gate778'. This condition commences the operation of a timing device 780',which after a certain lapse of time produces an output through OR gate781' to reset the flip-flop 772'. The resetting of the flip-flop 772'causes the AND gate 774' to conduct, thereby increasing the rate of gainof the amplifier 763' to the medium tracking rate. Upon transfer from asingle HRSG operation to a two HRSG operaton, the gain of the amplifier763' is changed to a fast rate in response to a digital output on theinput 766', and the resetting of the flip-flop 772' through the singlegenerator operation negative input 766'.

In summary, when both HRSG's are in operation, the pressure can slideupwardly or downwardly depending on the output of the generators withoutthrottling the turbine inlet valves provided that the pressure is abovethe predetermined low limit. Should the pressure get below the minimum,then an output signal on TPL causes the control valve to the steamturbine to close, thereby building up the pressure in the steamgenerators at least to the lower limit. During the two HRSG operation,the integrator 752' remembers the actual pressure on the input 748' ofthe comparator 747' by the previously described circuit. Referencesetpoints for increasing and decreasing the signal to the amplifier 763'are selected to be slightly below the generated setpoint of the throttlepressure control system of the digital computer in order to preventinteraction between the hardware and the programmed digital computersystem. Thus, when both HRSG's are in operation, the pressure ispermitted to slide at any position along the line BF, as shown in FIG.5.

Upon the removal of one HRSG from operation, in response to a steamgenerator trip (for example); or, should one of the generators HRSG haveits superheat outlet steamflow fall to below 30 % of its maximum, thepressure reference on line 750' is switched by the transfer device 745',and the output of the function generator 744' is substituted therefor asthe input to the comparator 747'. The characteristic output of thefunction generator 744' is controlled by the steam flow from theparticular generator HRSG which is still in operation. The transferdevice 764' is switched to the integrator 752' so that the actualpressure of the steam generators prior to the transfer to the onegenerator operation is substituted as the pressure reference. Upontransfer from two to one HRSG in service, the pressure would tend tofall very rapidly without any throttling action of the turbine controlvalve until the pressure flow relationship reaches line GE. However, thesubstitution of the flow from the steam generator in service accordingto the predetermined characterization of the function generator 744'together with a change in the rate of gain of the amplifier 763' to aslow transfer rate provides a throttling action for controlling the rateof decay of the pressure flow until it reaches the line GE of FIG. 5. Inother words, the pressure is controlled to decay gradually between thecurve BF and GE when transferring to a one HRSG operation. The steamcontrol valves for the turbine would continue to close in response to anoutput on TPL until such time as the pressure flow relationship is onthe line segment GE of FIG. 5. When the input 768' is equal to the inputon 751' for a predetermined length of time, then the input lines 776'and 777' have no potential; and the gain of the amplifier 763' isswitched to the medium operational rate for the one HRSG operation andthe control valve will close or hold position to keep the pressure flowrelationship on the line GE.

Briefly, upon transfer from two to one operation, the stored trackedoutput is applied to the comparator, and since the actual pressure,following the pressure prior to transfer is greater, and a TPL outputoccurs to throttle the valves. The substituted pressure flowrelationship is of course less than the actual pressure to the input ofthe comparator, which applies a decreasing signal to the amplifier.Inasmuch as the rate of gain of the amplifier, is slow, the feedbackfrom the output of the integrator is slow to shut off the signal fordecreasing the pressure reference for cutting the output from TPL. Oncethe output of the integrator and the pressure reference obtained fromthe flow are equal, the control valves hold position, and a medium rateof gain is applied to the integrator. Then, the control valves willcontinue to close as required to maintain the pressure flow along thecurve GE. Any opening of the control valves must be accomplishedmanually. Of course, when the pressure reaches the 300 pound minimum,throttling occurs through the limiting device 766'.

Referring to FIGS. 15A and 15B, a functional diagram of the controlsystem for the steam turbine control valves and main bypass valves,which in the present embodiment is implemented by a programmed digitalcomputer, includes a speed monitor logic function referred to as 700B'.The speed monitor logic function includes an input V3050 which is theproper speed under normal conditions. It includes an overspeedprotection controller input V3051 and a supervisory speed input V3052.The speed monitor function logic 700B' provides an output V3987 whichrepresents the actual speed of the turbine. A select operating modefunction 700C' is the logic for a particular selected operational state,such as coordinated control, operator automatic, or operator analog.This logic function 700C' provides a demand signal V3993, which iseither automatically selected or selected by the operator at the maincontrol panel. A reference demand algorithm 700F' performs the functionof determining the rate at which the demand speed for the turbine (speedcontrol mode) approaches and equals the reference speed of the system.The output of the reference demand algorithm 700F' is a referencerepresentation V3992.

A function block 700D' has an input V4072 representing the system flowat the outlet of the superheater from the heat recovery steam generatorHRSG1; and an input V5072 which represents the steam flow from the steamgenerator HRSG2. The function 700D' provides a representation V3966which is a function of flow according to a curve BF when both steamgenerators HRSG1 and HRSG2 are in service; and in accordance with curveCGE when only one of the generatiors HRGS1 and HRGS2 are in service.Recalling the description of FIG. 5, the curve BF of 700D' is slightlyabove the natural pressure flow relationship of the line correspondingto 2HRSG operation of FIG. 5. The line CGE of 700D' is slightly abovethe corresponding line for a single HRSG operation. Thus, the outputrepresentation V3966 is a representation of a pressure flow relationshipwhich depends on the rate of steam generation; that is, the rate ofgeneration for a single HRSG or for both HRSG's.

A throttle pressure monitor function 700E' has an input V4081 whichrepresents the steam header outlet pressure for the steam generatorHRSG1; an input V5081 which represents the outlet pressure for the steamgenertor HRSG2. Additionally, an input V3080 represents the throttlepressure to the steam turbine. The output of the throttle pessuremonitor 700E' is a representation V3965 which by way of the throttlepressure monitor function provides a representation of actual throttlepressure for the system.

Thus, the system includes a selected monitoring speed V3987, a selectedthrottle pressure V3965 and a selected speed demand V3993. Additionally,it provides a representation of pressure as a function of flow. Asumming device 700G' compares the selected speed V3987 with thereference speed V3992 provided that the breaker flip-flop L3966indicates that the main circuit breaker is open. The output of afunction 700G' is a speed error representation V3984 which is input to aproportional plus integral controller 700H'. The controller output V3986is multiplied at function 700J' by a constant which represents an outputranging gain for speed control K3988. The controller output V3996 isthen checked for a valve position limit V3949 by a function block 700K'.The controller output is then again put to a valve test function 700L'to provide outputs V3974 and V3973 which are representative of valveposition setpoints for the upper control valve and the lower controlvalve position respectively. The position setpoint representations areapplied to their respective NHC cards referred to as 700N' through aready output function 700M'. An output V3975 represents the NHC cardposition for the lower control valve; and output V3976 represents theNHC card position for the upper steam turbine control valve. These twooutputs are tracked by a function T713' which is connected to thereference representation V3992 to insure that the NHC card is in itsproper position.

When the breaker flip-flop L3N66 indicates that the main generatorcircuit breaker is closed, the speed reference signal becomes a loadreference signal; and it is input to a megawatt loop flip-flop L3008. Ifthe megawatt loop is out of service, the reference representation V3992is multiplied at function 700B' by the rated megawatts and thepercentage representation therefrom V3996 is applied to the function700K' and finally to the NCH card 700N' as described in connection withthe operation of the turbine under speed control. The function block700P' provides the megawatt compensated output V3996 in order that therewill be sufficient steam to minimize motoring action upon closure of themain circuit breaker. A megawatt feedback loop is also provided which iscut in or out by a megawatt flip-flop L3008. The actual megawatt outputis subtracted from the load reference V3992 at function 700Q', and theoutput error is divided by the rated megawatts K3990 to provide on inputrepresentation V3988 to the proportional plus integral controller 700R'.An output V3990 of the controller 700R' is multiplied by the loadreference V3992; and the compensated megawatt output therefrom V3991 isconverted to a percentage of rated megawatts by the function 700P'.

Thus, the steam turbine control valves both during wide range speedcontrol and load control are operated toward their open position duringstartup and load increase in accordance with representations relating toa reference speed or a reference load as compared to the actual speedduring speed control and as compared to actual megawatts during loadcontrol.

The output V3966 from the function 700D' is ramped by function T700' andapplied to a difference function T701'. The selected throttle pressureV3965 is also applied to the difference function T701' to generate arepresentation of error V3967. When the actual selected throttlepressure V3965 is greater than the output V3966 of the function 700D'the error output V3967 is positive, and in a direction to open thebypass valve. When the actual throttle pressure is less than the outputV3966, the error is negative and in a direction to close the bypassvalve. A function T704' has a central deadband to prevent the bypassvalve from oscillating in response to small deviations in the inputsV4072 or V5072 and is characterized by a steep slope on the outside ofthe deadband to operate the bypass valve quickly in the appropriatedirection in response to larger error changes. An error input V3960 tothe proportional plus integral controller T705' results in an outputV3962 which is multiplied by rated pressure at function T706' to obtaina representation corresponding to a percentage of rated pressure V3963.This is applied to a summing device T710' and the NHC card T712' for themain bypass valve through a ready output function T711'. Throttlepressure runup logic T707' is provided to open the bypass valve througha ramp function T708' with its output V3964 applied to the summingdevice T710' to increase the valve setpoint V3963 to open the bypassvalve in response to certain predetermined contingencies. Initial bypassvalve logic cracks the bypass valve slightly when starting up tocompensate for mechanical characteristics of the valve.

The error representation V3967 is applied to a summing function T702'for summing with a characterized output from function T702A', whichprovides an input V3969 to throttle pressure runback logic T703'. Theoutput of T702A' is a function of the pressure representation V3965. Therunback logic T703' determine the condition of certain system stateswhich prevent an output L3999 for decreasing the speed/load referenceV3992. The curve function of T720A' prevents an unnecessary runback ofthe control valve, that is, at low pressures a small deviation of thepressure will cause the error V3967 to be varied; and at higherpressure, a larger deviation is permitted.

In operation, during the building up of steam pressure, with both thebypass and control valves closed, the bypass valve will start to openwhen the pressure has reached the minimum requirement. The bypass valvecontinues to open in accordance with the error output representationV3967 as the rate of steam generation increases as determined by theflow representation V4072 and V5072. If such flow inputs indicates thatthere are two HRSG's in service, then the function 700D' operatesaccording to the curve BF (see FIG. 5). If there is only one HRSG inservice then the function 700D' selects the curve CGF (see FIG. 5). Thecurve BF is slightly higher than the natural pressure flow relationshipof the steam with the control valves wide open; that is, when thepressure flow relationship is on the line BF, the control valves wouldnot be completely open; and when the control valves are wide open, thepressure flow relationship is slightly below the curve BF. The curve CGEis similarly determined, except that the steam inlet is throttled over aportion of the flow, and this throttling determination for the controlvalves is less than the curve CGE.

Assuming that both HRSG's are in service, should the pressurerepresentation V3965 become less than the characterized representationV3977, then the bypass valve will be operated toward its closed positionuntil such time as the error signal V3967 indicates that the pressureflow relationship is along the line BF.

Assuming that the system is in operator automatic condition, theoperator selects the demand speed V3993 which produces a referencerepresentation V3992. The reference representation and the speed monitorrepresentation V3987 provide the error representation V3984. As long asthe reference representation is greater than the speed monitorrepresentation the control valves will continue to operate towards anopen position. During this time, the bypass valve is being controlled bythe error representation V3967 to operate towards a closed positon tomaintain the pressure flow relationship along the curve BF. Thus, as thecontrol valves open to bring the steam turbine up to speed by the bypassvalve continues to close to maintain the proper pressure flowrelationship. When the steam turbine is synchronized at 3600 rpm, thebreaker flip-flop L3966 is closed and the control valves continue toopen as the load demand is increased. Once the control valves are fullyopen, the bypass valve is fully closed. At this time, the control tomaintain the pressure flow relationship is transferred to the steamcontrol valves, because the bypass valve is attempting to maintain thepressure flow relationship along the line BF. This maintains the bypassvalve closed because the selected actual throttle pressure V3965 is nowslightly less than the characterized representation V3977 and the errorsignal is in a valve close direction. Any further reduction in thepressure flow relationship only increases the error signal in a valveclose direction. The error V3967 is summed with the output from thedynamic function T702A' so that small deviations of high pressure do notresult in an output TPL. Should the pressure flow relationship fallbelow the line BF (FIG. 5). substantially as determined by the curve ofT702A', the throttle pressure runback logic function T703' decreases thereference representation V3999 which operates the control valves throughtheir respective NHC card 700N' towards a closed position. The controlvalves will continue to operate towards a closed position until suchtime as the error representation V3967 is insufficient to overcome theoutput of T702A'.

The throttle pressure runup logic T707' and the function T704' controlsany opening of the bypass valve to prevent such opening unless thepressure and/or system conditions warrant such operation. The sequenceduring acceleration and loading of the turbine is the closing of thebypass valve as the control valves open with the bypass valve solelycontrolling the pressure flow relationship. Once the bypass valve isclosed, the pressure flow relationship is controlled by the controlvalves. Should the pressure flow relationship decrease, when the bypassvalve is closed, the control valve will fully close before the bypassvalve assumes control of the pressure flow relationship.

Therefore, the pressure can increase or decrease along the line BF ofthe function 700D' depending on the loading of the turbine. Any decreasein the pressure flow relationship will cause the control valves tooperate towards their closed position until such relationship is back onthe line BF. When only one HRSG is in operation, the function 700D'controls the bypass valve to its closed position along the line CGE inthe same manner as described for the control along the line BF.

FIGS. 16 through 22 inclusive are flow charts including the various linefunctions briefly described in connection with FIG. 15. The legends usedin such flow charts are described in Appendix A herein; and aninspection of such flow charts together with the Appendix should providea clear description of the logic utilized without a detaileddescription.

Referring to FIGS. 7A, 7B, and 7C and NHC card 801U' and an NHC card801L' is provided to control the upper and lower steam inlet controlvalves respectively. Although under all conditions of operation theupper and the lower control valves of the steam turbine are operated inwhat is usually termed single valve mode; that is, the position of eachof the valves is identical and they are moved toward the open of theclosed position simultaneously; there is provided an individual NHC cardfor each of the valves in the system. Each of the NHC cards areidentical, and a general description is given of their overall functionin connection with a description of FIG. 8.

When the system is transferred from automatic to manual control, the NHCcard responds to the analog hardware inputs instead of the inputs fromthe digital computer. Specifically, binary output data from the computeris input to gates referred to at 802' for operating a register control803' to operate a register counter 804' in either an upwardly ordownwardly direction. The register counter 804' provides its output to adigital to analog converter 805' which provides an analog output whichvaries from 0 to +10 volts depending upon the particular count in theregister. The output of the digital to analog converter 805' isconnected to a servo mechanism 806' for operating the upper steam inletcontrol valve.

In the operator analog or manual mode, the counter 804' is increased aslong as there is a signal present on input 810'. The counter 804' isdecreased as long as there is a signal present on input 811'. A clockpulse generator 812', determines the rate at which the count in theregister is effective. A signal at input 813' of the clock 812' providesa normal rate of count increase. A signal at input 814' of the clock812' provides a fast rate of increase.

A mode control function 815' is used to transfer the NHC cards between amanual and an automatic mode; or in other words, between the state ofbeing responsive to the digital computer or to the analog hardware. Themode control function 815' puts the NHC card in an automatic conditionin response to a distinctive signal on its input 816'. The input isprovided by closing a contact to operate a signal conditioner 817' (FIG.7A) in response to the operation of the "automatic" pushbutton on theBTG board. In response to the distinctive signal on the input 816' forautomatic operation, a signal is generated on 818' to produce a computerinterrupt to initiate automatic operation, provided, that the system isin a condition therefor. When a distinctive signal is present on input829' of the mode control 815', the NHC card is transferred tpo manualrendering it responsive only to the signals on the inputs 810' or 811'of the register control 803'. Either the operation of the manualpushbutton on the BTG board to close a contact to signal conditioner821' (FIG. 7A) or a signal generated in response to a computer powerfailure by signal conditioner 822' provides the distinctive signal oninput 820' for transferring the NHC card to manual. When the NHC card isa "manual" condition, a ready function block 823' provides an output on824' to indicate to the operator that the NHC cards are in a manualcondition. Additionally, the NHC crd 801U' is transferred to manual inresponse to the presence of a distinctive signal on input 825' to theregister control 803'. When the mode control function 815' is in amanual condition, a signal is present on output 826' which is utilizedin the logic circuitry for increasing the count of the register control803' in response to a signal on its input 810' through AND gate 827'(FIG. 7A).

The output TPL of the comparator 765' described in connection with FIGS.6A and 6C is connected by input 828' (FIG. 7A) through an OR gate 829'(FIG. 7B) to provide a signal on the input 811' of the register control803' to decrease the register counter 804'. Thus, as described inconnection with FIGS. 6A, 6B and 6C when a signal is present on theinput 828', either the pressure from the common header of the heatrecovery steam generators HRSG is below the predetermined minimum, orthe velocity of the steam is excessive in accordance with apredetermined pressure flow relationship. Also, a signal on the input828' of the TPL output is connected through an OR gate 830' to morerapidly operate the clock 812' by way of its input 814'. Also, the input828' is connected to an OR gate 831' to turn the clock 812' on throughits input 813' if it is not already on. Thus, if the flow pressure curverelationship should fall below that relationship set for the digitalcomputer portion of the control, the output on TPL or the input 828'permits the pressure to decay gradually as described in connection withFIGS. 6A, 6B and 6C by decreasing the count of the register in acontrolled manner. If desired the "go" logic may require a manualoperation to open the control valves after an output has occurred onTPL.

The NHC card 801L' for operating the lower steam inlet control valve isidentical to the previously described NHC card 801U'; and the variousfunctional components therein have been referenced with similarreference numerals including the suffix L. Although, there is an NHCcard for each control valve, the inputs to the NHC cards described inconnection with 801U' are parallel connected to the NHC card 801L' sothat they operate simultaneously in an identical manner. The remaininginputs in logic circuitry shown in connection with FIGS. 7A, 7B and 7Care provided for operating an NHC card under certain operatingcontingencies and operator demands. Such logic is evident from aninspection of the Figures and will not be described herein.

Referring to FIGS. 9A, 9B and 9C, the analog hardware system foroperating the main steam bypass valve also includes an NHC card 801B'which operates in a manner identical to the previously described NHCcards 801U' and 801L' for the pper and lower steam control valves. underautomatic control, an input gate 802B' provides data to operate aregister control 803B' for counting a register 804B' up or down. Acounting up of the register counter 804B' opens the main bypass valveand a down count closes the main bypass valve proportional to suchcount. The output from the counter 804B' is input to a digital to analogcounter 805B' to generate in its output 840' an analog voltage which isconverted to a current by a voltage current converter 841'. The outputof the voltage to current converter 841' operates to main bypass valve.The register control for manual or analog hardware includes an input842' for increasing the register count of the counter 804B' through theregister control 803b'. An input 843' to the register control 803B'decreases the count of the register counter 804B'. The clock 812B'determines the rate at which the register counter 804B' will count ineither an up or a down direction. A mode control function 815B' is usedto change the mode from manual to automatic or vice versa in accordancewith a distinctive signal input 844' or 845'.

Also, the output of the mode control function 815B' when in the manualmode provides a distinctive signal on 846' which is used in the logic topermit the main bypass valve to be operated toward an open position.Similarly, a signal on either 842' or 843' to increase or decrease thecounter 804B' via the register control 803B' transfers the NHC card tomanual control.

The bypass valve NHC card and its associated analog circuit also servesto switch the steam inlet control valves to manual through theirassociated NHC cards 801U' and 801L' over an input 850' (FIG. 9B). Ifthe pushbutton for the main bypass manual valve is operated, the modecontrol 815B' is switched to manual and an input 851' is connected to ORgate 852' to simultaneously set the main steam inlet control valves tomanual. Also, the manual pushbutton for the bypass valve provides aninput to an OR gate 853' which changes the mode control function 815B'to manual. The OR gate 853' has another input for setting the mainbypass valve to manual through its mode control input 844' which setsthe steam control valve to manual through its output 850' by way of anAND gate 855' which will conduct through the OR gate 852' when theoperator automatic pushbutton from input 856' is not operated ascontrolled by a negative function block 857'.

In the manual mode, should the pressure from the HRSG's exceed apredetermined maximum, as determined by a high signal monitor 858' theregister control 803B' is operated to increase the register counter byway of its input 842' through an AND gate 860' and an OR gate 861'. TheAND gate 860' determines that the NHC card is in manual through theoutput 846' of the mode control 815B'; and also determines that the mainheader pressure from the HRSG's is not below the predetermined minimumdetermined by a negative function 862'. The negative function 862' doesnot provide an input to the AND gate 860' to permit the valve to beopen, if a low signal monitor 863' provides an input ot an OR gate 864'which operates to decrease the register counter through an AND gate865'. Should the pressure drop below the predetermined minimum of 300pounds, for example, in the present embodiment, the OR gate 864' passesa signal through AND gate 865' to the input 843' of the register control803B' to decrease the register counter 804B' and operate the main steambypass valve toward a closed position. The clock 812B' is controlled byan input 866' which in turn is controlled by a summing function 867'. Anoutput from the high signal monitor 858' or the low signal monitor 863'causes an OR gate 870' to conduct to operate a switch or transferfunction 871' for introducing a value to the summing device 867' foroperating a clock at a slow rate. The clock 812B' is operated at a fastrate through the summing device 867' by a transfer device or switch 872'which is operated by inputs on a line 873' . The switch 872' is operatedfrom such input 873' in response to contingency conditions which wouldrequire a fast closing operation of the bypass valve.

FIGS. 10A through 10L, which illustrate in detail the analog circuitryof the throttle pressure limiting system described in connection withFIGS. 6A and 6B, include reference characters identical to those inFIGS. 6A through 6C. These reference characters correspond to similarlyreferenced functions shown in FIGS. 6A through 6C; and an understandingof FIGS. 10A through 10L can be had by referring to the description inconnection with FIGS. 6A through 6C and the reference charactersincluded therein. Similarly, FIGS. 12A through 12F, which illustrate indetail the analog circuitry for controlling the steam turbine inletcontrol valves include reference characters identical to those in FIGS.7A through 7C. These reference characters correspond to similarlyreferenced functions described in connection with FIGS. 7A through 7C;and an understanding of FIGS. 12A through 12F can be held by referringto the description of FIGS. 7A through 7C and the reference therein.FIGS. 13A through 13F, which illustrate in detail the analog circuitryfor the main steam bypass valve control system include referencecharacters identical to those in FIGs. 9A through 9C. These referencecharacters correspond to similarly reference functions described inconnection with FIGS. 9A through 9C; and an understanding of FIGS. 13Athrough 13F can be had by referring to the description of FIGS. 9Athrough 9C and the reference therein.

It is to be understood that more than two steam generators may beutilized in a combined cycle plant, or one steam generator withadditional capacity may be used. Also, a predetermined pressure flowrelationship for limiting the throttle pressure may be used in certainillustrations when both steam generators are in service.

Also, modifications may be made in the control system without departingfrom the spirit or scope of the inventions as defined by the claims.

FLOW CHART LEDGENDS FOR FIG. 16 THRU FIG. 22

Fig. 16a

v4072 hrsg1 flow

v5072 hrsg2 flow

v3080 throttle pressure

v3966 throttle pressure set point ramp input

v3967 throttle pressure error for bpv

v3969 throttle pressure error for cv

v4081 hrsg1 sh. outlet pressure

v5081 hrsg2 sh. outlet pressure

v3965 selected throttle pressure

v3977 throttle pressure set point ramp output

l3976 manual ov

l3999 throttle pressure runback

l3225 hold button lamp

l3986 computed hold

l3166 throttle pressure ann. co

l3194 hrsg flow transducer lamp

l3195 throttle pressure transducer lamp

l3191 hrsg sh. pressure transducer lamp

l3962 all pressure transducers failed

k3981 boiler minimum flow

k3880 one-boiler flow-pressure curve

k3870 two-boiler flow-pressure curve

k3976 throttle pressure error bias

k3984 throttle pressure ramp rate

700b select speed

fig. 16

v4072 hrsg1 flow

v5072 hrsg2 flow

v3080 throttle pressure

v3966 throttle pressure set point ramp input

v3967 throttle pressure error for bpv

v3969 throttle pressure error for cv

v4081 hrsg1 sh. outlet pressure

v5081 hrsg2 sh. outlet pressure

v3965 slected throttle pressure

v3977 throttle pressure set point ramp output

l3976 manual ov

l3999 throttle pressure runback

l3225 hold button lamp

l3986 computed hold

l31666 throttle pressure ann. co

l3194 hrgs flow transducer lamp

l3195 throttle pressure transducer lamp

l3191 hrsg sh. pressure transducer lamp

l3962 all pressure transducer lamp

k3981 boiler minimum flow

k3680 one-boiler flow-pressure curve

k3870 two-boiler flow-pressure curve

k3976 throttle pressure error bias

k3984 throttle pressure ramp rate

700b select speed

fig. 17

v3999 valve position limit

v3998 accumulated vpl increment

l3239 valve position limit raise button/lamp

l3240 valve position limit lower button/lamp

k3999 valve position limite increment

k3998 maximum valve position limit

fig. 19a

v3050 main turbine speed

v3051 opc speed

v3052 supervisory speed

v3987 selected speed

v3992 reference

v3997 valve test signal

v6979 start timer

v3104 mw

l3966 breaker flipflop

l3162 speed transducer failure

l3972 request for auto sync

l3976 manual cv

l3190 upper cv test

l3189 lower cv test

l3164 close upper sv

l3165 close lower sv

l3185 sync speed monitor lamp

l3963 auto startup

l6035 sync speed status lamp

l3196 speed transducer status lamp

l3197 mw transducer status lamp

k3986 speed error deadband

k3982 sync speed

7005 valve statys monitor

fig. 19b

v3050 main turbine speed

v3051 opc speed

v3052 supervisory speed

v3987 selected speed

v3997 valve test signal

v6979 start timer

v3104 mw

l3966 breaker flipflop

l3162 speed transducer failure

l3972 request for auto sync

l3976 manual cv

l3190 upper cv test

l3189 lower cv test

l3164 close upper cv

l3165 close lower sv

l3185 sync speed monitor lamp

l3963 auto startup

l6035 sync speed status lamp

l3196 speed transducer status lamp

l3197 mw transducer status lamp

k3986 speed error deadband

k3982 sync speed

7005 valve status monitor

fig. 20

v3993 operator demand

v3992 reference

v3987 selected speed

v3985 speed controller integral output

v3984 speed controller last input

v3976 upper cv nhc card output

v3975 lower cv nhc card output

l3966 breaker flipflop

l3099 turbine latched ci

k3990 rated mw

k3988 speed control output ranging gain

7009 steam turbinr speed/load control

fig. 21

v3996 control valve set point

v3899 valve position limit

v3992 reference

v3991 nw compensated reference

v3980 nw controller total output

v3989 nw controller integral output

v3998 nw controller last input

v3987 selected speed

v3986 speed controller total output

v3985 speed controller integral output

v3984 speed controller last input

v3974 upper control valve set point

v3973 lower control valve set point

v1104 nw ai

v3983 demand

l3979 valve position limiting

l3222 nw flipflop

l3966 breaker flipflop

l3226 go button/lamp

l3986 computed hold

l3113 opc test permissive switch ci

l3114 opc acting ci

k3990 rated nw

k3998 speed control output ranging gain

k3950 speed controller parameters

k3940 nw controller parameters

7007 steam turbine ready/output

fig. 22

v6879 startup timer

l3976 manual cv

l2976 gt2 manual fuel valve

l1976 gt1 manual fuel valve

l6066 plant auto lamp

l3229 auto button

l3962 start up in coordonated CONTROL

A00b nw in/out logic

aoo9 auto sync logic

a007 ats logic

a00d go/hold logic

d059 upper cv test

d058 lower cv test

901f coord logic

ad19 ats ready logic

what we claim is:
 1. A combined cycle electric power plant comprising atlest one gas turbine, means for generating steam in response to heatenergy from the gas turbine, a steam turbine driven by steam supplied toit from the steam generating means, means for generating electric powerunder the driving power of the turbine, a plurality of valve means tocontrol the admission of steam to the turbines, a digital computer andan analog control system for operating the valve means, said digitalsystem including means to generate repetitive output pulses in responseto input voltages depending on plant conditions including steampressure, said analog system including means to generate an output pulseof a variable duration depending on input conditions including steampressure, a hybrid interface unit responsive selectively to either therepetitive output pulses of the digital system and the output pulse ofthe analog system to generate an analog representation having a valuedepending on the duration of the selected applied pulses, and controlmeans for each valve means governed by the value of the analogrepresentation to position its respective valve means.
 2. A combinedcycle electric power plant according to claim 1 further comprising meansresponsive to a predetermined contingency to render the interface unitresponsive to the analog system generated output pulse at times when theinterface unit is responding to the digital system output pulses.
 3. Acombined cycle electric power plant according to claim 1 wherein aninterface unit is provided for each valve means, and further comprisesmeans connecting electrically the analog output to at least a portion ofthe interface units in parallel, whereby each parallel connectedinterface unit responds similarly to the analog output pulse.
 4. Acombined cycle electric power plant as set forth in claim 1 wherein aninterface unit is provided for each valve means, a portion of saidinterface units being electrically independent of each other, andfurther comprises means to apply the output pulses of the digital systemto each of the electrically independent interface units concurrently,and means to vary the output pulses of the digital system for one of theinterface units depending on the position of the valve means for anotherof said units.
 5. A combined cycle electric power plant as set forth inclaim 1, further comprising means in the analog system responsive topredetermined contingencies to vary the operation of the interface unitin response to the duration of the analog output pulse.
 6. A combinedcycle electric power plant as set forth in claim 1, further comprisingmeans responsive to the output of each interface unit when the interfaceunit is responsive to the analog system output pulse to generate thedigital repetitive output pulses for the interface, whereby upontransfer to the digital system, the repetitive output pulses provide theidentical interface output.
 7. A combined cycle electric power plant asset forth in claim 1, further comprising means connecting the analogsystem to the interface unit when the interface is responsive to thedigital system to operate the interface commencing at the cessationlevel of the digital output upon transfer to the manual system.
 8. Acontrol system for a plurality of valve means to govern the admission ofsteam to a steam turbine power plant, comprising a digital computer andanalog control portion, said digital computer control portion includingmeans to generate repetitive output signals in response to a referencesignal, said analog control portion including means to generate anoutput signal having a variable duration in response to a referencesignal, a hybrid interface unit for each valve means, each interfaceunit being responsive selectively to the digital output signals and theanalog output signal to generate an interface output signal the value ofwhich varies depending on the input, a valve operating means to positioneach valve means in response to the interface output signal, and meansto transfer the response of the interface unit between the analog andthe digital output portion.
 9. A control system according to claim 8wherein a portion of the interface units are electrically connected inparallel to respond equally to the selected digital or analog signals,and a portion are electrically independent, and further comprises meansto vary the reference signal for one interface unit as governed by thevalve position of a valve operating means connected to the output of anelectrically independent interface.